Preheat the oven to 350°F (175°C).
Lightly grease a 6-cup muffin tin with olive oil.
In a large bowl, whisk together the eggs, whole milk, salt, and black pepper until fully combined and slightly frothy.
Add the shredded cheddar cheese, diced red bell pepper, diced green bell pepper, diced onion, and chopped spinach to the egg mixture. Stir gently to combine.
Divide the egg mixture evenly among the 6 muffin cups, filling each about three-quarters 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 the muffins from the oven and let them cool for 5 minutes before carefully removing them from the tin.
Serve warm or at room temperature.