Preheat the oven to 375 degrees F and line a baking sheet with parchment paper. Combine the walnuts, melted butter, brown sugar, cumin, paprika, cayenne and 1/4 teaspoon salt in a medium bowl; toss, then spread the nuts on the prepared baking sheet. Bake until the nuts are golden brown, about 10 minutes. Stir, then set aside to cool.
Meanwhile, make the dressing: Whisk the yogurt, lemon juice, honey, rice vinegar and a pinch each of salt and black pepper in a small bowl; set aside.
Trim the tops and bottoms of the grapefruits, then use a paring knife to remove the peel and white pith. Cut between the membranes to release the segments into a large bowl.
Add the spiced walnuts, celery, endive, arugula and chives to the bowl with the grapefruit. Add the yogurt dressing, season with salt and pepper and toss.
Photograph by Christina Holmes
Recipe courtesy of Food Network Magazine