Preheat the oven to 350 F.
In an oven safe saucepan, sauté onions and mushrooms with olive oil until soft.
In the meantime, whisk the eggs and add cheese to the mixture. Season with a dash of salt and pepper.
Incorporate the mixture with the cooked chicken sausage into the saucepan. Let it cook for few minutes.
Pour the eggs and cheese mixture into the saucepan to cover the rest of the ingredients.
Under medium heat, let the eggs cook for 3-5 minutes before placing inside the oven.
Place the frittata in the oven, bake for 10 minutes or until the eggs are fully cooked.