Heat olive oil in a skillet over medium heat, then sauté the diced onions and bell peppers until softened and fragrant, about 3–4 minutes.

Add chopped spinach to the skillet and cook just until wilted, then remove from heat and let the mixture cool slightly.

In a large bowl, whisk the eggs and milk together until smooth and slightly frothy.

Stir in salt, black pepper, and garlic powder until evenly distributed.

Fold the sautéed vegetables and shredded cheddar cheese into the egg mixture, ensuring everything is evenly combined.

Pour the mixture evenly into the prepared muffin tin, filling each cup about ¾ full.

Bake for 18–22 minutes, or until the centers are set and the tops are lightly golden.

Allow the muffins to cool in the pan for 5 minutes before gently loosening them with a knife and transferring to a rack.