Preheat a large skillet or griddle over medium heat until a drop of water sizzles on contact.
In a medium bowl, combine the mashed sweet potatoes, black beans, both cheeses, red onion, cilantro, cumin, chili powder, garlic powder, and salt, mixing gently with a spoon until all ingredients are evenly distributed.
Divide the sweet potato and bean mixture equally among the 4 tortillas, spreading the filling over one half of each tortilla and leaving a 1/2-inch border around the edge, then fold the other half over to create a half-moon shape and press down gently.
Brush the top of one folded quesadilla lightly with a portion of the olive oil and carefully place it oil-side down onto the preheated skillet, then brush the top side with more oil.
Cook for 3 to 4 minutes per side, pressing down occasionally with a spatula, until the tortilla is golden brown and crispy and the cheese inside is visibly melted.
Transfer the cooked quesadilla to a cutting board, repeat the cooking process with the remaining quesadillas, and let each one rest for 1 to 2 minutes before slicing into wedges.
