Preheat the oven to 350°F and lightly grease a muffin tin.
In a mixing bowl, whisk together the eggs, diced bell peppers, tomatoes, spinach, mushrooms, salt, and pepper.
Pour the egg mixture evenly into the muffin tin, filling each cup about 3/4 full.
Bake for 15-20 minutes or until the egg muffins are set and slightly golden.
Allow them to cool for a few minutes before removing from the muffin tin.
Serve warm or refrigerate for a grab-and-go breakfast option.