Place the cubed sweet potatoes in a microwave-safe bowl with a splash of water. Cover and microwave for 5 to 7 minutes until tender. Drain any excess water and set aside.
Heat the olive oil in a large skillet over medium heat. Add the diced onion and cook for 3 minutes until softened.
Stir in the black beans, cumin, chili powder, salt, and pepper. Add the cooked sweet potatoes and mash lightly with a fork to combine. Cook for another 2 minutes, then remove the filling from the skillet.
Wipe the skillet clean and return it to medium heat. Place one tortilla in the skillet and sprinkle with cheese. Spoon about a quarter of the filling over the cheese and top with more cheese. Add a second tortilla on top.
Cook for 2 to 3 minutes per side until the tortilla is golden and the cheese melts. Repeat with the remaining tortillas and filling.
Slice the quesadillas and top each serving with sliced avocado.