Total Time:  20 mins      Servings:  4

Ingredients

Tartar Sauce

• ¼ cup mayonnaise
• ¼ cup sour cream
• 3 tablespoons minced shallot (from 1 medium shallot)
• 2 tablespoons minced fresh flat-leaf parsley
• 1 ½ tablespoons drained prepared horseradish
• 1 ½ tablespoons  fresh lemon juice
• 1 ½ tablespoons minced cornichons (about 4 small cornichons)
• 1 tablespoon drained capers, chopped
• ¼ teaspoon kosher salt
• ¼ teaspoon black pepper

Sandwiches

• 1 teaspoon hot paprika
• 1 teaspoon garlic powder
• 1 teaspoon minced fresh oregano
• 1 teaspoon minced fresh thyme
• 1 teaspoon kosher salt
• ½ teaspoon black pepper
• 4 (4- to 5-ounce) skinless haddock fillets (about 3/4 inch thick)
• ¼ cup plus 2 teaspoons unsalted butter, divided
• 4 brioche hamburger buns, split
• 1 tablespoon  canola oil
• Butter lettuce leaves, for serving

Directions

Make the tartar sauce:

1. Whisk together mayonnaise, sour cream, shallot, parsley, horseradish, lemon juice, cornichons, capers, salt, and pepper in a small bowl. Refrigerate until ready to use.

Make the sandwiches:

1. Stir together paprika, garlic powder, oregano, thyme, salt, and pepper in a small bowl. Sprinkle spice mixture evenly over both sides of fish fillets. Set aside.
2. Heat a large cast-iron skillet over high. Spread 1 teaspoon butter over cut side of each bun half. Place buns, buttered side down, in hot skillet; cook until well toasted and browned, about 1 minute. Transfer to a work surface. Do not wipe skillet clean.
3. Add oil and remaining 2 tablespoons butter to skillet; cook over high until butter is melted. Add fish fillets; cook until blackened on bottoms, about 3 minutes. Flip fillets; cook until blackened on other sides and just cooked through, 2 to 3 minutes. Remove from heat, and transfer fish to a plate.
4. Spread 1 tablespoon tartar sauce over toasted side of each bun half. Arrange fish and lettuce evenly on bottom bun halves. Cover with top bun halves, toasted side down. Serve alongside remaining 1/2 cup tartar sauce at the table.