1. Measure juice from citrus salsa. If needed, add orange juice to make 1/3 cup. Place fillets of sole in a heavy zip-lock plastic bag and pour in juice. Seal bag and chill for 15 minutes.
2. Meanwhile, in a blender or food processor, pulse cashews until finely ground. In a shallow bowl, mix nuts with panko, salt, and pepper.
3. Pour 2 tablespoons oil into a 10- to 12-inch nonstick frying pan over medium-high heat; when hot, lift fillets from juice. Dip each in egg, then in cashew mixture to coat. Working in batches, place fillets in a single layer in pan (do not crowd). Cook until browned on the bottom, about 2 minutes; turn with a wide spatula and cook until other side is browned and fish is opaque but still moist-looking in center (cut to test), about 2 minutes longer. Drain briefly on paper towels; keep warm in a 200° oven. Repeat to fry remaining fish, wiping out pan with paper towels and adding 2 tablespoons oil between batches.
4. Transfer fillets to plates and top with citrus salsa; serve at once with remaining salsa alongside.