In a medium bowl, whisk the eggs with salt and pepper until well combined.
Heat the butter in a non-stick skillet over medium heat. Once melted, add the diced onion and bell pepper. Sauté for about 3-4 minutes until softened.
Pour the whisked eggs into the skillet with the vegetables. Cook, stirring gently, until the eggs are just set, about 2-3 minutes. Remove from heat.
Place one tortilla in the skillet over low heat. Sprinkle half of the shredded cheese on one half of the tortilla. Spoon the scrambled egg mixture over the cheese, then top with the remaining cheese.
Fold the tortilla in half to cover the filling. Cook for about 2-3 minutes on each side, or until the tortilla is golden brown and the cheese is melted.
Remove from the skillet and let it cool for a minute before slicing into wedges. Serve with sour cream and salsa if desired.