In a skillet, heat olive oil over medium heat. Add diced onions and bell peppers, and sauté for about 3-4 minutes until softened. Season with salt and pepper.
In a bowl, whisk the eggs and pour them into the skillet with the vegetables. Scramble until fully cooked, about 3-5 minutes. Remove from heat.
On one half of each tortilla, layer half of the cheese, followed by the scrambled egg mixture, sausage or bacon, and the remaining cheese. Fold the tortillas in half.
Wipe the skillet clean and return it to medium heat. Place the folded quesadillas in the skillet and cook for 3-4 minutes on each side, or until golden brown and the cheese is melted.
Remove from the skillet and let cool for a minute before slicing into wedges. Serve with sour cream and salsa if desired.