In a medium bowl, whisk together the eggs, milk, salt, and pepper. Set aside.
In a skillet, melt 1 tablespoon of butter over medium heat. Add the onions and bell peppers and cook until softened, about 5 minutes. Add the cooked breakfast sausage, if using, and cook for an additional minute.
Pour the egg mixture into the skillet and stir constantly until the eggs are cooked through, about 5-7 minutes. Remove from heat.
In a separate skillet, melt the remaining tablespoon of butter over medium heat. Place one tortilla in the skillet and sprinkle with half of the shredded cheese.
Spoon half of the egg mixture on top of the cheese. Top with the other tortilla and press down gently.
Cook until the cheese is melted and the tortilla is crispy, about 2-3 minutes. Carefully flip the quesadilla over and cook for an additional 2-3 minutes on the other side.
Remove the quesadilla from the skillet and let it cool for a minute or two. Cut into quarters and serve with salsa and sour cream.
Repeat the process with the remaining ingredients to make the second quesadilla.
Enjoy your delicious breakfast quesadilla!