1. 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.
2. Preheat grill to medium heat.
3. Combine the remaining 1 tablespoon mustard and honey in a small bowl, and set aside.
4. Combine arugula, onion, juice, and oil in a medium bowl. Set aside.
5. 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.