Preheat the oven to 350 degrees.
Crack 4 eggs into a mixing bowl and whisk together until thoroughly mixed.
Finely chop the pepper, mushrooms, and onion.
Heat a frying pan over medium heat. Add teaspoon of olive oil and chopped vegetables to the frying pan. Cook until vegetables are soft and onions are translucent, roughly 5-7 minutes. Set aside.
Insert cupcake liners into the muffin tin cups you plan to use OR Spray muffin tin cups with non-stick cooking spray.
Mix cooked vegetables into scrambled egg mixture.
Using an ice cream scoop (if you have one), portion one and a half scoops of egg mixture into the four muffin cups.
Cook for 20 minutes at 350. Remove tin from oven, eggs should still be very slightly runny on top. Sprinkle cheddar cheese on top of each egg muffin. Put back in oven.
Finish cooking for 5 minutes.
Remove tin from oven, serve, and enjoy!