Fried Fish Sandwiches with Cucumbers and Tartar Sauce

Preparation

1. Tartar Sauce

   Step 1

   Mix mayonnaise, lemon zest and juice, relish, capers, mustard, and garlic in a small bowl to combine; season with salt.

2. Fish and assembly:

   Step 2

   Prepare a grill for medium-high heat. Whisk Old Bay seasoning, baking powder, cayenne, 1/2 cup flour, and 1/2 cup cornstarch in a medium bowl. Whisk remaining 1/4 cup flour and 1/4 cup cornstarch in another medium bowl.

   Step 3

   Set a deep cast-iron skillet or large Dutch oven fitted with thermometer on grill; pour in oil to come 1" up sides. Heat until thermometer registers 375°F.

   Step 4

   Meanwhile, thinly slice cucumbers lengthwise. Toss in a third medium bowl with vinegar and gently massage with your hands to soften them slightly. Let sit, tossing occasionally, at room temperature at least 10 minutes.

   Step 5

   Lightly brush insides of rolls with butter and grill just until toasted, about 30 seconds. Transfer to a plate.

   Step 6

   Gently whisk egg and beer in a small bowl to combine, then gently whisk into Old Bay mixture in 3 additions. Working in 2 batches, season fish lightly with salt. Dredge in flour mixture, knocking off any excess, then dip into batter and lift up with a fork, letting excess drain off and wiping against side of bowl to eliminate any thick drips. (Batter should completely coat the fish without looking clumpy.)

   Step 7

   Carefully lower fish into oil and fry, turning halfway through, until deeply browned and an instant-read thermometer inserted into the thickest part of each piece registers 125°F–130°F, 7–9 minutes. Using a slotted spoon, transfer to a wire rack set inside a baking sheet.

   Step 8

   Drain most of vinegar from cucumbers and toss in dill. Build sandwiches with fried fish, cucumbers, lettuce, and tartar sauce.

3. Do Ahead

   Step 9

   Cucumbers (without dill) can be pickled 2 days ahead. Cover and chill.