Preheat the oven to 350°F (175°C) and liberally grease a 12-cup muffin tin with olive oil.

In a large mixing bowl, whisk the eggs and milk together until the yolks and whites are fully incorporated.

Season the egg mixture with salt and black pepper.

Fold in the chopped spinach, diced red bell pepper, and shredded cheddar cheese.

Distribute the mixture evenly across the 12 muffin cups, filling each approximately three-quarters full.

Bake for 20 minutes or until the eggs are set and a toothpick inserted into the center comes out clean.

Allow the muffins to cool in the pan for 5 minutes before using a thin spatula to release them from the edges.