For the dressing, combine oil, 3 tablespoons grapefruit juice, mustard and blend of whisk. Season with salt and pepper. Place arugula in a large bowl. Add fennel. Segment the other grapefruit and add to bowl. Toss with dressing and serve immediately. This is one salad that keeps nicely if not dressed. So if you think you wont eat it all, remove a portion before dressing.