Preheat oven to 375F.

In medium size bowl, using a fork, mix together eggs, pepper, and salt. Stir in cheese and set aside. Heat 12-inch non-stick, oven safe skillet pan over medium high heat. Add butter to pan and melt add in onion cook for about a minute until the onion is soft. Add asparagus and mushroom to pan and saute for 2 to 3 minutes.

Pour egg mixture into pan and stir with rubber spatula just to ensure that the mixture is evenly distributed on the bottom of pan. Cook for 1 minute or until the egg mixture is starting to set on the bottom, then put the skillet in the oven. Watch the frittata as it cooks. Let it cook in the oven for 10-12 minutes until the eggs are set and lightly browned.