1. Heat 1/4 in. oil in a large frying pan over medium-high heat. Fry onions in 2 batches until light golden and crisp, turning once with tongs, about 5 minutes per batch. Drain on paper towels.
2. Meanwhile, whisk together citrus juices, soy sauce, and mirin in a small bowl.
3. Pour off all but a film of oil from frying pan. Fry fish in batches until cooked through, 2 minutes per side.
4. Transfer fish to plates and pat off oil with a paper towel. Drizzle with some citrus sauce and top with spring onions. Serve with remaining sauce.
*Find fat-bulbed spring onions (regular onions picked young) at farmers' markets and Latino markets.