To a 9- or 10-inch (medium) nonstick skillet, heat 1 tablespoon oil over high heat until the oil glistens and is very hot. Add the zucchini and sauté, tossing regularly, until browned. Transfer the zucchini to a plate and set aside.

To the same skillet, add the remaining 1 tablespoon oil over high heat. Add the onion and sauté, tossing regularly, until golden and fragrant. Add the garlic and asparagus, and cook until garlic is fragrant (this will take less than a minute). Add the spinach and dill and toss into the mixture. Remove from the heat.

Off the heat, add the zucchini to the asparagus mixture in the pan. Take the smoked salmon and tear off bite-sized pieces - tuck these pieces into the zucchini-asparagus mixture all around in the pan.

In a bowl, whisk together the eggs, egg whites, mustard, milk, salt, and pepper. Carefully pour egg mixture over the veggies and salmon in the pan. Top with the cheese.

Place pan back on the burner over medium heat. Cook until the egg around the sides of the pan start to firm and turn opaque. Put the pan in the oven and bake for 15 minutes or until set in the center and golden on top. Remove from the oven. Let frittata sit 4-5 minutes before using a rubber spatula to gently coax out of the pan onto a serving plate (if desired).