Preheat the oven to 375°F (190°C).
In a large bowl, whisk the eggs and milk together. Season with salt and pepper.
Stir in the cooked lentils, spinach, bell pepper, and onion.
Heat olive oil in an oven-safe skillet over medium heat.
Pour the egg mixture into the skillet and cook for about 5 minutes, until the edges begin to set.
Transfer the skillet to the preheated oven and bake for 25 minutes, or until the frittata is fully set and golden on top.
Remove from the oven and let it cool slightly before slicing and serving.