Combine onion, pickles, and pickle brine in a small bowl. Cover and chill for at least 1 hour or overnight.
Mix mayonnaise, dill, hot sauce, pepper, 2 tsp. lemon juice, and 1/2 tsp. salt in another small bowl. Cover and chill for at least 1 hour or overnight.
Place eggs, panko, and flour in 3 separate shallow medium bowls. Season each fish fillet with 1/2 tsp. salt. Working one at a time, dredge fish in flour, shaking off excess. Then, coat evenly with egg, allowing excess to drip back into bowl. Next, coat with panko, pressing to adhere. Transfer fish to a plate.
Heat oil in a large skillet over medium-high heat. Fry fish until breading is golden brown and fish is cooked through, 2–3 minutes per side. Transfer fish to a paper towel-lined plate. Sprinkle fillets with additional salt.
Spread a heaping tablespoon of the dill sauce on one side of each slice of bread. Pile each bottom slice with ½ cup lettuce, 1 fried fish fillet, and 1/4 cup pickled onion mixture. Top sandwich with second slice of bread, dill sauce side down, and press down gently. Transfer to a platter. Repeat with remaining sandwiches. Serve with lemon wedges and potato chips on the side.
*Tip: if using fresh-caught or whole fish, check out this tutorial for steps on how to clean, skin, and fillet a trout.
Recipe adapted from: www.epicurious.com