1 cup mayonnaise
2 teaspoons finely minced onion
½ teaspoon fresh lemon juice
1 tablespoon well-drained capers
2 teaspoons caper juice
2 dashes Tabasco sauce
1 tablespoon chopped chipotle
Dash of paprika for color
In a food processor with the steel blade, blend together the mayonnaise, onion, lemon juice, capers, caper juice, and Tabasco. Add the chipotle and mix with a couple of pulses. Mix in the paprika. Refrigerate until needed.
½ cup all-purpose flour
½ cup cornstarch
½ teaspoon salt
½ teaspoon baking powder
¾ cup beer (preferably pilsner or other light flavored beer)
2 qts. Vegetable oil for frying
4 (4-6 oz) skinless haddock or cod fillets, 1-inch thick
4 brioche buns, toasted
4 leaves Bibb lettuce
Set wire rack in rimmed baking sheet. Add oil to large Dutch oven until it measures about 1 ½ inches deep and heat over medium-high heat to 375 degrees. Pat fish dry with paper towels and transfer to batter, tossing gently to coat. Using fork, remove haddock from batter, 1 piece at a time, allowing excess batter to drip back into bowl; add to hot oil, briefly dragging fish along surface of oil to set batter before gently dropping into oil. Ad just burner, if necessary, to maintain oil temperature between 350 and 375 degrees.
Cook, stirring gently to prevent pieces from sticking together, until deep golden brown and crispy, about 4 minutes per side. Using a spider skimmer or slotted spoon, transfer fish to the prepared rack.
Divide tartar sauce evenly among bun bottoms, followed by fish and lettuce. Cover with bun tops.