Prep the filling: In a bowl, lightly mash the black beans with a fork so some are creamy and some stay whole.
Stir in red onion, jalapeño, cumin, chili powder, lime zest, and half the lime juice. Season with salt and pepper. Fold in cilantro.
Smash the avocados: In a separate bowl, mash the avocados with the remaining lime juice and a pinch of salt.
You want a spreadable texture, not a purée.
Assemble: Lay out the tortillas. Spread a thin layer of avocado on half of each tortilla. Spoon the black bean mixture over the avocado.
Top with shredded cheese. Fold each tortilla over to make a half-moon.
Heat the pan: Warm a large skillet over medium heat. Add a light coating of oil or a small pat of butter.
You want the tortilla to crisp, not burn.
Cook the quesadillas: Place 1–2 folded tortillas in the skillet. Cook 2–3 minutes per side, pressing gently with a spatula, until the tortilla is golden and the cheese is melted.
Finish and repeat: Transfer to a cutting board and let rest 1 minute before slicing into wedges. Repeat with remaining quesadillas, adding oil as needed.
Serve: Plate with salsa, sour cream or Greek yogurt, extra lime wedges, and hot sauce.
A handful of fresh cilantro on top is a nice touch.