Heat grill to medium high.
Grill the meatballs, turning frequently, until cooked through to 165°F.
Transfer the meatballs to a plate and allow to rest.
While the meatballs are cooking, shave the fennel on a mandolin or with a sharp knife.
Combine the fennel, grapefruit segments, onion, mint, and olive oil. Season with salt and pepper to taste.
Transfer the fennel salad to a large bowl, top with meatballs, and serve immediately.