01 - Separate onion slices into individual rings. Set up three bowls: mix flour, smoked paprika, garlic powder, and salt in the first; whisk eggs in the second; place panko breadcrumbs in the third.
02 - Dredge each onion ring in the seasoned flour mixture, shaking off excess. Dip into the beaten eggs, then press firmly into panko breadcrumbs to coat evenly on all sides.
03 - Heat 2 inches of vegetable oil in a deep skillet over medium-high heat to 350°F. Fry onion rings in batches for 2-3 minutes until golden brown and crispy. Transfer to paper towels to drain.
04 - Heat a non-stick skillet over medium heat. Place one tortilla in the pan. Sprinkle evenly with 1/2 cup cheddar cheese and 1/4 cup mozzarella cheese.
05 - Arrange fried onion rings over the cheese layer. Drizzle with 2 tablespoons sour cream and sprinkle with chives if using. Place second tortilla on top.
06 - Cook for 2-3 minutes until bottom tortilla is golden brown. Carefully flip and cook another 2 minutes until cheese is completely melted and second side is golden. Repeat with remaining ingredients.
07 - Let quesadillas rest for 1 minute, then cut into wedges. Serve immediately with salsa, guacamole, and additional sour cream on the side.