Preheat the oven to 175 degrees Celsius (350 degrees Fahrenheit) and apply non-stick cooking spray to a standard 12-cup muffin tin.

Whisk the eggs, whole milk, salt, and black pepper in a large bowl until the mixture is homogenous and slightly aerated.

Distribute the chopped spinach, shredded cheese, and diced bell peppers evenly across the 12 muffin cups.

Pour the egg mixture over the solid ingredients in each cup until they are approximately 75 percent full.

Bake for 20 to 22 minutes or until the egg centers are set and the surfaces are lightly golden.

Remove from the oven and allow to cool in the tin for 5 minutes to facilitate easy removal.

Extract muffins using a silicone spatula and store in an airtight container for up to 4 days refrigerated.