In a small bowl, combine the mayonnaise with the sun-dried tomatoes, gherkins, red onion, dill, and lemon juice. Season the tartar sauce with salt and cayenne.

In a shallow bowl, whisk the eggs with the water. In another shallow bowl, mix 1/2 cup of the flour with salt and cayenne. In a third shallow bowl, combine the potato flakes with the remaining 1/4 cup of flour and season with salt and cayenne. Line a baking sheet with wax paper and another one with paper towels.

Working in batches, dredge the cod in the flour, tapping off the excess. Dip the cod in the egg, allow any excess to drip back into the bowl, then coat the cod in the potato flakes, pressing to help them adhere. Transfer the cod to the wax paper-lined baking sheet.

In a large, deep skillet, heat 1 1/2 inches of oil to 325 degrees. Working in 2 batches, fry the cod in the hot oil, turning once, until deep golden and crispy, about 3 minutes. Using a slotted spoon, transfer the fish to the paper towel-lined baking sheet and sprinkle lightly with salt. Repeat with the remaining fish. Serve immediately with the tartar sauce and lemon wedges.

The sauce can be refrigerated for 1 week.