In a medium bowl, combine the mashed sweet potatoes, black beans, red onion, cilantro, cumin, chili powder, and garlic powder until well mixed, then season with salt to taste.
Lay four tortillas flat on a clean work surface and evenly distribute the sweet potato mixture among them, spreading it into an even layer while leaving a 1/2-inch border around the edges.
Sprinkle 1/4 cup of shredded Monterey Jack cheese evenly over the filling on each tortilla, then top with the remaining four tortillas to form quesadillas.
Heat 1 tablespoon of olive oil in a large non-stick skillet or griddle over medium heat until shimmering, then carefully place two quesadillas in the skillet.
Cook for 3-4 minutes per side until the tortillas are golden brown and crispy and the cheese is fully melted, pressing down gently with a spatula halfway through cooking.
Transfer the cooked quesadillas to a cutting board, then repeat with the remaining tablespoon of oil and two quesadillas. Let the quesadillas rest for 2 minutes before slicing each into four wedges using a sharp knife or pizza cutter for clean cuts.
