Spray indoor ridged grill with nonstick cooking spray; heat over medium heat. Or, prepare outdoor grill according to manufacturer's directions. Sprinkle salmon on both sides with tarragon, salt and black pepper. Grill 5 minutes on each side, until fish is cooked through. Meanwhile, to prepare sauce, in small nonstick skillet, heat oil. Add garlic; cook, stirring constantly, 2 minutes. Stir in broth, relish, lemon juice and red pepper; cook, stirring frequently, 2-3 minutes, until sauce boils and reduces slightly. Remove from heat. To assemble sandwich, divide lettuce and tomato evenly over bread. Place salmon on top and drizzle evenly with sauce.