Cut the top and bottom off the grapefruit. Stand up the fruit and slice off the rind and white pith underneath, reserving the rind so you can add the juice to the salad. Cut the fruit into segments, reserving any juice that accumulates. Set aside.
Add the orange juice and half of the mint leaves to a large bowl. Season with salt and pepper, then drizzle in the oil.
Quarter the fennel bulbs and trim the core. Using a mandoline or a chef's knife, thinly shave each quarter. Add the fennel and Hungarian peppers to the vinaigrette. Finely chop the reserved fennel fronds and add to the bowl. Fold in the grapefruit segments and their juices and the remaining mint leaves. Squeeze in the juice from the reserved grapefruit top and bottom and toss to combine. Serve.