Grilled Sumac Chicken Thighs with Parsley Salad

Name: Grilled Sumac Chicken Thighs with Parsley Salad
Ingredients: 4 1/2 teaspoons ground sumac; 2 teaspoons ground cumin; 1/4 teaspoon ground cinnamon; 8 skin-on, bone-in chicken thighs (about 3 pounds total); Kosher salt and freshly ground pepper; 1/4 cup extra-virgin olive oil; 2 tablespoons fresh lemon juice (from about 1 lemon); 3 cloves garlic, finely chopped; 1 large red onion, cut into 1/2-inch-thick rounds; 3 cups fresh parsley leaves (from 1 large bunch); 1 cup thinly sliced radishes (6 to 8); Grilled flatbread and hummus, for serving