Heat 1 tablespoon of oil in a 10-inch nonstick skillet. Add the onion and garlic; cook until lightly golden, 1 to 2 minutes. Add the beans, corn and another tablespoon of oil if the pan seems dry. Season with the salt, cumin and pepper, and stir in the cilantro. Let cook over medium-low heat until the beans and corn are heated through, about 5 minutes.
Heat a 10-inch cast-iron skillet over medium heat.
Meanwhile, arrange the tortillas flat on a counter. Sprinkle 1 1/2 tablespoons of cheese on one half of each tortilla. Spoon 2 tablespoons of the bean-corn mixture over the cheese. Top with another 1 1/2 tablespoons of cheese. Fold the filled tortillas into half-moons.
Place 2 quesadillas in the skillet. Cook until golden on the bottom and the cheese has started to melt, 2 to 3 minutes. Use a spatula to flip the quesadillas, and cook until golden on the other side, 2 to 3 more minutes,. Repeat with the remaining quesadillas.
Serve topped with pico de gallo if desired.
Easy Swap In: We usually make the filling with leftover grilled or roasted corn, but frozen corn works great too. Don't thaw the corn-just add the frozen kernels to the pan with the black beans and allow 1 to 2 extra minutes for them to heat through.