Preheat the oven to 180°C (350°F).

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

Heat olive oil in a large oven-safe skillet over medium heat. Add the onions and bell peppers, and sauté for 5-7 minutes, until softened.

Add the spinach and cook for another 2-3 minutes until wilted.

Pour the egg mixture over the vegetables and cook for 3-4 minutes until the edges start to set.

Sprinkle the grated cheese on top, if using, and transfer the skillet to the preheated oven.

Bake for 10-12 minutes, or until the center is set and the top is golden.

Garnish with fresh herbs if desired and serve warm.