Heat olive oil or butter in a skillet over medium heat.
Add onion and bell pepper and cook until softened, about 3–4 minutes.
Add spinach and cook just until wilted. Transfer vegetables to a plate.
In a bowl, whisk eggs with salt and black pepper.
Lower heat to medium-low and scramble eggs gently until just set. Remove from heat.
Lay a tortilla flat and spread eggs over half of it.
Top with cooked vegetables and a light layer of cheese.
Fold the tortilla over the filling.
Cook in a clean skillet over medium heat until golden and crisp on both sides.
Remove, rest briefly, slice, and serve warm.
