Step 1

Mix mayonnaise, cornichons, mustard, lemon juice, and sugar in a small bowl; season with salt and pepper.

Step 2

DO AHEAD: Tartar sauce can be made 2 days ahead; cover and chill.

Step 3

Preheat oven to 400°. Place fish on a rimmed baking sheet and rub with oil; season generously with salt and pepper. Roast until fish flakes easily, 8–10 minutes; let cool. Using a fork, break fish into large pieces; set aside.

Step 4

Whisk egg, mayonnaise, and lemon zest in a large bowl. Add fish, corn, chives, and ½ cup panko; season with salt and pepper and mix gently to combine. Gently form ¼-cupfuls of mixture into ¾”-thick patties (you should have 12). Chill on a parchment-lined baking sheet until firm, 30–35 minutes.

Step 5

Place remaining 1½ cups panko in a shallow bowl; season with salt and pepper. Coat patties in seasoned panko, pressing gently to adhere. Heat vegetable oil in a large skillet over medium-high heat. Working in batches, cook patties until golden brown, about 4 minutes per side. (If panko begins to burn between batches, wipe out skillet and start with new oil.) Transfer patties to a wire rack and season with salt.

Step 6

Serve with tartar sauce and lemon wedges for squeezing over.

Step 7

DO AHEAD: Patties can be formed (but not coated in panko or cooked) 12 hours ahead. Cover and chill.