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.