Spicy Squash Quesadillas

Ingredients

• a 3/4-pound seedless piece butternut squash (, peeled and cut into 3/4-inch dice (about 5 cups))
• 1 medium onion (, unpeeled, cut into eighths)
• 1 large garlic clove (, unpeeled)
• 1 cayenne pepper (, minced, seeds removed)
• 1 tablespoon vegetable oil
• eight 5- to 6-inch flour tortillas
• 1 cup cooked black beans
• 1 cup coarsely grated cheddar or swiss cheese
• 1/2 stick ((1/4 cup) unsalted butter, softened)