For best results, choose salmon from the meaty center of the fish and avoid portions cut from the tail section (where the body starts to taper). Tail-end flesh contains more connective tissue than center-cut fillets. Use black sesame seed buns, if available.
Place 1/4 pound salmon, 1 tablespoon mustard, and rind in a food processor; process until smooth. Spoon puree into a large bowl. Place remaining 3/4 pound salmon in food processor; pulse 6 times or until coarsely chopped. Fold chopped salmon, tarragon, shallots, salt, and pepper into puree. Divide mixture into 4 equal portions, gently shaping each into a 1/2-inch-thick patty. Cover and chill until ready to grill.
Preheat grill to medium heat.
Combine the remaining 1 tablespoon mustard and honey in a small bowl, and set aside.
Combine arugula, onion, juice, and oil in a medium bowl. Set aside.
Lightly coat both sides of burgers with cooking spray. Place patties on a grill rack; grill 2 minutes. Carefully turn patties, and grill for an additional 1 minute or until desired degree of doneness. Place 1 patty on bottom half of each bun; top each serving with 1 1/2 teaspoons honey mixture, 1/4 cup arugula mixture, and top half of bun.