Preheat the oven to 375°F (190°C).

In a mixing bowl, whisk together the eggs, milk, salt, and pepper until well combined.

Stir in the spinach, bell pepper, onion, and cheddar cheese.

Heat a skillet over medium heat and add a little oil or butter.

Pour the egg mixture into the skillet and cook for about 5 minutes until the edges start to set.

Transfer the skillet to the preheated oven and bake for 15-20 minutes, or until the frittata is set in the center.

Remove from the oven, let cool slightly, then slice and serve.