How to Make It
Heat a large cast-iron skillet over medium heat. Add 1 tablespoon butter to skillet and cook until just melted. Add onion to skillet and cook, stirring occasionally, until soft and translucent, about 6-8 minutes. Transfer onions to a plate; wipe skillet clean.
Lay tortillas flat on a clean work surface. Sprinkle 1/4 cup cheese on 1 half of each tortilla. Divide turkey and cooked onions evenly over tortillas, then top each evenly with remaining cheese. Fold tortillas in half, creating a half-circle.
Heat 1 tablespoon butter in cast-iron skillet over medium heat. Add 2 quesadillas to skillet and cook until golden, about 3 minutes per side. Transfer quesadillas to a cutting board and repeat with remaining tablespoon of butter and 2 quesadillas. Cut quesadillas into 4 wedges to serve. Serve with desired accoutrements, such as sour cream, hot sauce, jalapeños, and cilantro.