1. Preheat oven to 400°. Cut asparagus into 1-inch pieces, discarding tough ends.
2. Sauté mushrooms and onion in 2 tsp. hot oil in a 10-inch nonstick ovenproof skillet over medium heat 4 to 5 minutes or until onion is tender; remove from skillet. Add remaining 1 tsp. oil to skillet, and sauté asparagus 2 to 3 minutes or until tender; stir in 1/4 tsp. each salt and pepper. Remove from skillet. Wipe skillet clean.
3. Melt butter in skillet over medium heat. Whisk together eggs and remaining 1/4 tsp. each salt and pepper. Add egg mixture to skillet. As eggs start to cook, gently lift edges of egg with a spatula, and tilt pan so uncooked portion flows underneath. Cook 2 to 3 minutes or until almost set. Top with vegetables and feta cheese.
4. Bake frittata at 400° for 16 to 18 minutes or until slightly browned and puffy. Serve immediately.