1. Pre-heat the oven to 350ºF. Spread the hazelnuts onto a baking sheet. Toast for about 10 minutes until golden brown and fragrant. Let cool and coarsely chop.
2. For the dressing, put the Dijon mustard, garlic, orange juice, and white wine vinegar in a bowl. Whisk in the olive oil. Season with salt and pepper.
3. Slice off the tops and bottoms of the grapefruits and oranges. Run the knife down the sides of each grapefruit and orange removing the peel and pith. Using a pairing knife, cut the fruit segments apart from in between the membranes.
4. In a salad bowl, toss together the arugula with the dressing (you may have some dressing left over, pour a little and then taste to see if you need to add more).
5. Divide the arugula among 4 plates, top each with the citrus segments, toasted hazelnuts, pomegranate seeds, and parmesan cheese.