- Preheat the oven to 375°F (190°C).
- In a large bowl, whisk together the eggs and milk until well combined.
- Add the chopped spinach, diced bell peppers, diced onions, salt, and pepper to the egg mixture.
- Heat olive oil in an oven-safe skillet over medium heat.
- Pour the egg mixture into the skillet and cook for about 5 minutes, or until the edges start to set.
- If using cheese, sprinkle it on top of the frittata.
- Transfer the skillet to the preheated oven and bake for 15-20 minutes, or until the frittata is fully set and lightly golden on top.
- Remove from the oven, let it cool slightly, then slice and serve.
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