01 - Separate onion slices into individual rings. Set up three shallow bowls: combine 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 flour mixture, shaking off excess. Dip into the egg wash, allowing excess to drip off. 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 without overcrowding, turning once, until golden brown and crispy, about 2-3 minutes per batch. Transfer to a wire rack or paper towels to drain.
04 - Heat a non-stick skillet over medium heat. Place one tortilla in the pan. Sprinkle with 1/2 cup cheddar cheese and 1/4 cup mozzarella cheese. Arrange fried onion rings evenly over the cheese. Drizzle with 2 tablespoons sour cream and sprinkle with 1/2 tablespoon chives.
05 - Place a second tortilla on top. Cook for 2-3 minutes until the bottom tortilla is golden brown and cheese begins to melt. Carefully flip and cook an additional 2 minutes until the second side is golden and cheese is completely melted.
06 - Transfer quesadilla to a cutting board. Let rest 1-2 minutes before cutting into 6 wedges. Serve immediately with salsa, guacamole, and additional sour cream on the side.