In a large pan set over medium heat, sauté the onions and garlic in the oil until the onions are translucent. Add the grated sweet potatoes, oregano, chili powder, cumin and cayenne and cook, covered — covering is important to prevent sticking — for about 10 minutes, stirring every few minutes. When the sweet potato is tender, add salt and pepper to taste and remove the filling from the heat. Squeeze 1/2 a lime over the sweet potatoes, mix, taste, and adjust with more lime and salt to taste.
Bake the quesadillas: Spread one-eighth of the filling and 2 tablespoons of the cheese on each tortilla. Bake in a 350ºF oven until the cheese is fully melted and quesadillas are hot, approximately 15 minutes.
Optional to cook the quesadillas stovetop: heat 2 teaspoons oil over medium heat. Fill tortillas lightly — less is more — with sweet potato filling and some cheese. Fold them in half. Cook two at a time until golden, 2 to 3 minutes. Flip. Cook until golden, about 2 more minutes. Repeat with remaining tortillas, cheese, filling, adding more oil as needed.
Serve with more fresh lime, sour cream, and salsa if you prefer. Enjoy!