Preheat your oven to 375°F (190°C).
In a large bowl, beat the eggs and milk together. Add salt and pepper to taste.
Heat the olive oil in an oven-safe skillet over medium heat. Add the chopped onion and bell pepper. Sauté until they start to soften, about 5 minutes.
Add the spinach and tomato to the skillet. Cook for another 2-3 minutes until the spinach is wilted.
Pour the egg mixture over the vegetables in the skillet. Stir gently to combine.
Sprinkle the shredded cheese evenly over the top, if using.
Transfer the skillet to the preheated oven. Bake for 15-20 minutes, or until the eggs are fully set and the top is golden.
Remove the frittata from the oven. Let it cool slightly, then slice and serve.