Combine first 6 ingredients in a small bowl; set aside.
Place 1 cup salad greens on each of 4 plates. Arrange grapefruit sections evenly over greens, and sprinkle evenly with green onions. Stir juice mixture until well blended; spoon dressing over salads.
Tip: Section the grapefruit over a bowl, then use the juice to make the dressing for this salad.