Grate 1/2 teaspoon rind from orange (see Step 1 below); set grated rind aside.
Peel oranges and grapefruit as described in Step 2 below. Cut out citrus sections over a bowl, reserving juice (see Step 3 below). Set citrus sections aside. Squeeze membranes over bowl to extract additional juice. Discard membranes.
Combine 6 tablespoons citrus juice, grated rind, olive oil, and next 4 ingredients in a small bowl; stir well with a whisk. Cover dressing, and chill. Reserve remaining citrus juice for another use.
Rub Jamaican jerk seasoning over salmon. Place salmon on a broiler pan coated with cooking spray. Broil 14 minutes or until fish flakes easily when tested with a fork. Flake salmon into bite-size pieces; cover and chill at least 30 minutes.
Combine citrus sections, spinach, and onion in a large bowl. Drizzle dressing over salad, and toss gently to coat.
Divide salad evenly among five plates; top each salad evenly with salmon, and sprinkle with sunflower seeds. Serve immediately.