PREHEAT oven to 450ºF. Juice 2 of the grapefruit; combine with sugar in small saucepan. Cook on medium-high heat 25 min. or until sugar is dissolved and juice is reduced by half. Pour grapefruit syrup into blender. Add cream cheese; cover. Blend until smooth; set aside. Keep warm.
LINE 15x10x1-inch baking pan with foil. Pour 1/4 cup of the dressing on prepared pan; place in oven 5 min. to heat. Remove from oven. Immediately place salmon, skin-side up, on pan; top with remaining 1/4 cup dressing. Bake 20 to 25 min. or until salmon flakes easily with fork, turning once after 12 min.
SEGMENT remaining grapefruit, combine with juices and onions. Serve salmon topped with warm sauce and grapefruit topping.