Heat a large skillet over medium heat.
Spread one side of each slice of bread with butter or margarine. Place the slices in the skillet and cook until golden brown on both sides.
Remove the bread from the skillet and place it on a plate. Top one slice with avocado slices, onion slices and queso fresco. Place the other slice of bread on top to form a sandwich.
Return the sandwich to the skillet and cook until golden brown on both sides and cheese is melted. Serve hot!.
