Brown hamburger and onion together. Drain and stir in taco seasoning mix.
Heat refried beans thoroughly.
Place a tortilla on a plate and spread with a medium-thin layer of refried beans.
Spread 1 heaping tablespoon of the ground meat on top of the beans.
Spoon on 1 tablespoon each of salsa and sour cream, then sprinkle with lettuce and cheese.
Place another tortilla on top and sprinkle with cheese.
Transfer quesadilla to a hot, cast iron skillet and place into a 400 degree oven until lightly browned and cheese is melted. Watch closely, as they can burn quickly!
Remove from oven and transfer to cutting board. Cut into 4 wedges, and serve with more salsa, guacamole, and sour cream as desired.
Hint: You can substitute cooked and shredded chicken for the hamburger.