How to Make It
Cook fregola in salted water according to package directions. Drain and rinse. Transfer to a large bowl, and let cool to room temperature, about 15 minutes.
Preheat broiler to high with oven rack 6 inches from heat. Toss ½ cup of the onion with ½ tablespoon of the oil and ¼ teaspoon of the salt on a baking sheet. Broil until lightly charred, 3 to 4 minutes; set aside.
Cut peel and pith from grapefruits using a sharp paring knife. Working directly over a medium bowl to catch juices, cut between membranes to release the grapefruit segments; set segments aside.
Whisk together lemon juice, honey, Dijon, pepper, 3 tablespoons of the grapefruit juice, 1 tablespoon of the dill, and remaining 4 tablespoons oil and 1½ teaspoons salt in a large bowl. Add fregola, charred and raw sliced red onion, feta, hazelnuts, and grapefruit segments; gently toss to coat. Garnish with remaining 2 tablespoons dill.