Preheat your oven to 375°F (190°C).
Lightly grease a 6-cup muffin tin with olive oil.
In a large mixing bowl, crack the 6 large eggs.
Add 1/4 cup whole milk, 1/4 teaspoon salt, 1/4 teaspoon black pepper, and 1/4 teaspoon baking powder to the eggs. Whisk together until the mixture is smooth and slightly frothy.
Stir in 1/2 cup shredded cheddar cheese, 1/4 cup finely chopped red bell pepper, 1/4 cup finely chopped green bell pepper, 1/4 cup finely chopped yellow onion, and 1/4 cup chopped fresh spinach until evenly combined.
Pour the egg mixture evenly into the prepared muffin tin cups, filling each about 3/4 full.
Place the muffin tin in the preheated oven and bake for 18 to 20 minutes, or until the egg muffins are set and lightly golden on top.
Remove from the oven and allow the muffins to cool in the tin for 5 minutes.
Carefully run a butter knife around the edges of each muffin and gently remove them from the tin.
Serve warm or at room temperature.