Heat oven to 400° F. In a medium bowl, combine the beans, corn, and salsa. Place 4 tortillas on a parchment- or foil-lined baking sheet.
Sprinkle the tortillas with half the cheese. Using a slotted spoon, top with the bean mixture. Sprinkle with the remaining cheese; top with the remaining tortillas.
Bake the quesadillas until the cheese has melted, 5 to 7 minutes. Transfer to a cutting board.
Meanwhile, in a large bowl, combine the onion, cilantro, salt, pepper, lime juice, and oil. Add the lettuce and toss.
Cut each quesadilla into 6 wedges. Serve with the salad.