Using a zester, remove 1/2 teaspoon of zest from one or two of the oranges and set aside. Cut away the peels from the oranges and grapefruit, leaving them free of any white pith. Working over a bowl, segment the oranges and grapefruit by cutting in between the membranes on both sides of each segment. Use the edge of the knife to help release the segments into the bowl. Repeat until the oranges and grapefruit are completely segmented. Squeeze the membranes over a separate nonreactive bowl to release any remaining juices and set aside.
Thinly slice the plum, fennel and red onion and add them to the bowl with the citrus segments. Add the fennel fronds and mint leaves to the bowl and toss well.
Combine the 1/4 cup blood or navel orange juice with the grapeseed oil, red wine vinegar, Dijon mustard, orange zest, sea salt and black pepper and whisk well. Toss the vinaigrette with the orange and grapefruit segments, plum and fennel mixture. Marinate for 15 minutes and set aside until ready to use.
Yield: 4 to 6 servings