Combine mayonnaise, chipotle, and lime zest and juice in bowl. Working in 3 batches, pulse salmon in food processor until chopped into ¼-inch pieces, about 2 pulses, transferring each batch to large bowl.

Add 2 tablespoons mayonnaise sauce and scallions to chopped salmon and gently knead with your hands until well combined. Lightly moisten your hands, divide salmon mixture into 4 equal portions, then gently shape each portion into 1-inch-thick patty. Place patties on parchment paper–lined rimmed baking sheet and refrigerate for 15 minutes.

3A. For a charcoal grill Open bottom vent completely. Light large chimney starter filled with charcoal briquettes (6 quarts). When top coals are partially covered with ash, pour evenly over grill. Set cooking grate in place, cover, and open lid vent completely. Heat grill until hot, about 5 minutes.

3B. For a gas grill Turn all burners to high, cover, and heat grill until hot, about 15 minutes. Leave burners on high.

Clean cooking grate, then repeatedly brush grate with well-oiled paper towels until black and glossy, 5 to 10 times. Sprinkle patties with salt and pepper. Using spatula, place patties on grill and cook

until well browned on first side and patties easily release from grill, 3 to 5 minutes. Gently flip patties and continue to cook until well browned on second side, centers are still translucent when checked with tip of paring knife, and burgers register 125 degrees (for medium-rare), 3 to 5 minutes.

Stack lettuce leaves together to create 4 lettuce wraps. Serve burgers on wraps, topped evenly with remaining mayonnaise sauce, tomato, and avocado.