Preheat the oven to 400°F.
Steam asparagus for about 5 minutes until tender; set aside.
Beat the eggs in a medium bowl; beat in the cottage cheese and mustard; set aside.
Lightly spray an ovenproof skillet with the olive oil spray.
Add mushrooms and onions, if using; cook over medium heat until tender.
Stir in the asparagus.
Pour egg mixture over the veggies and cook over low heat for about 5 minutes or until it bubbles slightly.
Bake uncovered for about 10 minutes or until set.
Garnish with tomato slices.