Preheat the oven to 350°F (175°C).
Whisk the eggs in a large bowl with salt and pepper to taste.
Spray a 12-capacity muffin tin with cooking oil (olive oil preferred).
Pour the whisked eggs into each muffin tin, filling each about halfway.
Mix the bacon, onions, peppers, and mushrooms in a bowl (optional) before evenly distributing them into each muffin tin over the eggs.
Top with shredded cheese.
Bake for 14-16 minutes, or until the eggs are set. Allow additional time for firmer eggs.
Cool completely before serving to prevent the edges from sticking to the muffin tin.