Preheat your oven to 375°F (190°C). Warm the olive oil in an oven-safe skillet over medium heat until shimmering.
Sauté the chopped onions and bell peppers until they soften and become fragrant, about 3-4 minutes.
Add the chopped spinach to the skillet and cook for another 1-2 minutes until wilted. Remove from heat and let cool slightly.
In a mixing bowl, whisk the eggs until smooth and slightly frothy. Season with salt and pepper to taste.
Stir the sautéed vegetables into the beaten eggs, then fold in the shredded cheese for added flavor and creaminess.
Pour the egg mixture back into the skillet, spreading it evenly with a spatula.
Transfer the skillet to the preheated oven and bake for 15-20 minutes, until the eggs are set and the top is lightly golden.
Remove from the oven and let cool for a few minutes before slicing into wedges.
Slice into squares or wedges and serve warm, garnished with extra cheese or herbs if desired.