Preheat oven to 350°F. Generously spray a non-stick or silicone muffin pan with nonstick cooking spray.
Heat olive oil into a non-stick frying pan over medium heat. Add the bell peppers and onion and saute for 2 to 3 minutes. Stir in spinach and mushrooms and cook an additional 2 minutes. Set aside.
Whisk the egg whites into a medium-sized bowl. Season with salt and pepper. You can then add the vegetable mixture with the egg whites, then pour the egg mixture into the muffin cups, leaving 1/4″ from the top. Alternatively, you could place a heaping teaspoon of the vegetables into each cup, followed by the egg whites.
Place the muffin tin in the preheated oven and bake for 20-25 minutes or until the egg whites are set and lightly golden on top.
Once done, remove the muffin tin from the oven and let the muffin cups cool for a few minutes.Carefully remove the muffin cups from the tin using a butter knife or small spatula. Serve the egg white muffin cups warm or at room temperature. They can be stored in an airtight container in the refrigerator for up to 4-5 days.
