Preheat an outdoor grill for medium heat and lightly oil the grate. Melt butter in a saucepan over medium heat. Add onion and garlic; cook and stir until soft, about 5 minutes. Remove from heat; stir in lemon juice and salt. Place scallops in a large bowl, and toss with butter mixture; let stand for about 2 minutes. Thread scallops onto skewers, or use a wire grill basket. Cook on preheated grill until scallops turn opaque, turning once, about 4 to 5 minutes. Return remaining butter mixture to saucepan; bring to a boil, then reduce heat to low and keep warm while scallops cook. Toss or brush over grilled scallops and serve. Yields six servings.