Cook the Eggs: In a mixing bowl, whisk together the eggs, salt, and pepper. Heat butter in a skillet over medium heat. Once the butter has melted, pour in the egg mixture and scramble gently until cooked through. Set aside.
Prepare the Quesadillas: Heat a clean skillet or griddle over medium-high heat. Place one tortilla on the skillet and sprinkle half of the shredded cheddar and mozzarella cheese evenly over the tortilla. Add a few spoonfuls of scrambled eggs, followed by the diced bell peppers, onions, and tomatoes. Top with avocado slices.
Grill the Quesadilla: Place another tortilla on top, then press down gently. Cook for 2–3 minutes on each side, or until golden brown and crispy, and the cheese has melted. Repeat the process with the remaining ingredients.
Serve: Remove the quesadillas from the skillet, and slice them into wedges. Serve with sour cream, salsa, and a sprinkle of fresh cilantro for garnish.