Preheat and Sauté: Preheat your oven to 375°F (190°C). Warm the olive oil in an oven-safe skillet over medium heat. Add the onion, bell pepper, and mushrooms, and sauté until tender, about 5-7 minutes.
Add Chicken: Add the chopped cooked chicken to the skillet and cook for another 2-3 minutes to warm through.
Prepare Egg Mixture: In a large bowl, combine the eggs, milk, salt, and pepper. Whisk until the mixture is smooth and slightly frothy.
Combine and Cook: Pour the egg mixture evenly over the vegetables and chicken in the skillet. If using, sprinkle the cheese over the top. Stir gently to combine everything.
Bake: Transfer the skillet to the preheated oven. Bake for 10-15 minutes, or until the eggs are fully set and the top is slightly golden brown.
Cool and Serve: Carefully remove the skillet from the oven. Allow the frittata to cool for a few minutes before slicing and serving. It pairs well with a side salad or roasted vegetables.