In a skillet over medium heat, add olive oil. Once hot, add diced onions and bell peppers. Sauté for about 3-4 minutes until softened. Season with salt and pepper.
In a bowl, whisk the eggs and season with salt and pepper. Pour the eggs into the skillet with the vegetables and scramble until fully cooked. Remove from heat.
Lay one tortilla flat on a clean surface. Sprinkle half of the cheddar cheese evenly over the tortilla.
Spread the scrambled egg mixture over the cheese, then add the crumbled sausage or bacon on top. Sprinkle the remaining cheese over the filling.
Place the second tortilla on top and press down gently.
Wipe the skillet clean and return it to medium heat. Carefully place the quesadilla in the skillet. Cook for about 3-4 minutes until the bottom is golden brown and the cheese begins to melt.
Carefully flip the quesadilla using a spatula and cook for another 3-4 minutes on the other side until golden brown and the cheese is fully melted.
Remove from the skillet and let it cool for a minute before slicing into wedges. Serve with sour cream and salsa if desired.