Asparagus Casserole has tender asparagus and mushrooms baked in a luscious Dijon cream sauce. This festive and colorful side dish is a variation of asparagus au gratin and perfect for a special dinner. Easy to make and so impressive in its simplicity.
Asparagus Casserole: A simple yet elegant side dish
Three of my favorite ingredients, asparagus, mushrooms, and Dijon mustard, together in a dreamy decadent sauce. This is Creamy. Delicious. Heaven. Next time you make a fancy meal, serve Asparagus Casserole on the side.
Growing up, my grandmother made the Exact Same meal every holiday. It was a delicious, but it was always the same. Once I set out on my own and learned to cook), one thing I started doing early on was mixing up the holiday meals.
So this Asparagus Casserole is in the spirit of Green Bean Casserole, only even better and something new.
Asparagus is my favorite vegetable. I am generally not a big veggie fan, but asparagus is one I eat weekly and I am good with that. Usually I roast it in the oven with a little olive oil and seasoning, because asparagus is delicious and does not need much help.
This is why this elegant side dish is a perfect way to prepare asparagus. We are not over doing it – we are not adding a ton of cheese or condensed soup. Just a couple easy ingredients and voila.
As I said above, all the ingredients here are simple and approachable.
- Fresh asparagus spears. You want to end up with 1 lb asparagus to cook after trimming off the woody ends. Therefore, if your asparagus spears are standard, 1-½ lbs will yield about a pound after trimming. If your spears are thicker, go up to 2 lbs because more will be lost to trimming.
- Extra virgin olive oil. Olive oil is used to brown the mushrooms, sauté the onion, and make the crunchy topping.
- Baby portabella mushrooms. If not available, substitute white button mushrooms.
- White onion. A yellow onion is an appropriate substitute.
- Heavy cream.
- Half and half.
- Whole grain mustard and Dijon mustard. I used both because I wanted the pearls of whole grain mustard as well as the more assertive flavor of Dijon.
- Seasonings: kosher salt and fresh cracked pepper.
- Crunchy topping: panko breadcrumbs, Parmigiano-Reggiano cheese, and a little bit of olive oil. These parmesan is what makes this recipe technically an asparagus au gratin.
- Fresh basil for garnish.
Steps for preparation
Asparagus Casserole is extraordinarily easy to prepare and can be assembled well in advance of cooking, even up to a day ahead.
Prepare the vegetables
Blanch the asparagus. For veggies, especially green veggies, to keep their color while cooking, we blanch them first. All blanching is is putting the asparagus in boiling water for a minute or two (depending on the thickness of the spears), then transferring directly to ice water to stop the cooking.
Be sure to drain the asparagus very well after the ice bath because we do not want to introduce water into the casserole and have it end up watery.
Cook the mushrooms and onions. This step is done in one skillet. Olive oil is heated and the mushrooms are browned. Then to the mushrooms, a little more olive oil is added and the onion is sautéed with the mushrooms.
Make the Dijon cream sauce
To the same skillet used above (wiped clean), cream and half and half are added, then simmered until thickened. Whisk in the whole grain mustard, Dijon mustard, salt, and pepper, and the sauce is done.
Assemble the casserole and bake
The asparagus, mushrooms, onion, and sauce are combined into a small casserole dish. Once topped with a mixture of panko breadcrumbs, parmesan, and olive oil, the asparagus au gratin (because we put cheese on top!) is baked until all golden, bubbly, and, most importantly, when the asparagus is fork tender.
This last part is the key. Thicker asparagus spears will take longer to cook while thin spears will cook faster. I have done both, and ideally spears that are not baby fine thin or thick like a stick – i.e. right in the middle – work best.
But either way, we make it work. If using thinner spears, blanch only 1 minute and bake the asparagus casserole around 20 minutes. If using thick spears, blanch 2 minutes and cook up to 30 minutes or until whenever they get tender.
Tips and FAQs
I go into more detail above, but understand that thin spears will cook faster and thick spears will take longer to cook.
Thick spears need to be trimmed more because a larger proportion of them will be “woody”. Therefore, adjust the amount purchased and cook time accordingly.
Because of the longer cook time, thick spears may not maintain their brilliant green color. Regardless, your asparagus au gratin will be amazing.
Be sure to drain the asparagus WELL after blanching. Meaning, you should drain, get all the ice gone, and get rid of as much water as possible. A salad spinner would probably help a lot if you own one (I do not). Point being that we do not want to add water and end up with a casserole in watery sauce.
Browning mushrooms properly takes a bit of technique. For tips, check out my tutorial How to Brown Mushrooms.
You may assemble this asparagus casserole up to a day in advance, and it reheats very well after cooking.
Looking for more holiday side dish recipes?
- The Best Stuffing Ever
- Grilled Green Bean Casserole with Bacon
- Horseradish Roasted Potatoes
- Wine and Thyme Macaroni and Cheese
- Holiday Butternut Squash
- Cranberry Chorizo Macaroni and Cheese
- Herbed Potato Gratin
- Oven Roasted Broccoli in Dijon Pesto Vinaigrette
- Cream Cheese Smashed Potatoes
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- 1-½ lb fresh asparagus spears see Notes
- 1-½ Tbsp extra virgin olive oil divided (plus ½ tsp for the topping)
- 8 oz baby portabello mushrooms sliced
- ½ medium white onion diced
- ½ cup heavy cream
- 1 cup half and half
- 1 Tbsp whole grain mustard
- 1 Tbsp Dijon mustard
- ½ tsp kosher salt
- good pinch fresh cracked pepper
- 2 Tbsp panko breadcrumbs
- 1 Tbsp freshly grated Parmigiano-Reggiano cheese
- garnish: fresh chopped basil leaves
- Heat oven to 400°F.
- Blanch the asparagus in boiling water and transfer immediately to ice water to stop the cooking. If asparagus spears are thin, blanch 1 minute. If they are thick, blanch 2 minutes. Drain completely.
- Heat 1 tablespoon olive oil in a medium saucepan over high heat. Add the mushrooms and cook, tossing occasionally, until mushrooms are well browned.
- Add half a tablespoon olive oil and the onion to the mushrooms. Cook, tossing occasionally, until onion is lightly browned. Remove from the heat and set aside.
- In a clean, medium skillet, heat the cream and half and half over medium heat until bubbly. Simmer gently 10 minutes to thicken. Whisk in the whole grain and Dijon mustard, salt, and pepper. Remove from the heat.
- In a mixing bowl, combine the asparagus, mushroom-onion mixture, and Dijon cream. Transfer to a 2-qt casserole dish or equivalent.
- In a small bowl, combine 1/2 teaspoon olive oil, panko breadcrumbs, and parmesan cheese. Sprinkle over the asparagus casserole.
- Bake at 400°F for 20-30 minutes until bubbly and golden on top, and asparagus is fork tender.
Calorie count is purely an estimate calculated using an online application to serve as a guide and not to be taken as accurate nutritional information.