Grilled Buttermilk Chicken With Summer Salad

Name: Grilled Buttermilk Chicken With Summer Salad
Ingredients: 1 garlic clove, finely grated; ¼ cup fresh lemon juice; 1 tsp. Dijon mustard; ½ cup extra-virgin olive oil, plus more for drizzling; 3 Tbsp. plus ½ cup buttermilk powder; 2½ tsp. sugar, divided; 1¾ tsp. Diamond Crystal or 1¼ tsp. Morton kosher salt, divided, plus more; 1 tsp. freshly ground pepper, plus more; 1 large English hothouse cucumber, halved lengthwise, sliced 1 thick on a diagonal; 1 lb. mixed tomatoes (such as beefsteak, heirloom, and/or cherry), cut into thick wedges, halved if small; 1 cup chopped dill and/or thinly sliced chives; 3 medium skinless, boneless chicken breasts (about 1½ lb.); 2½ tsp. garlic powder; 2 tsp. onion powder; 2 medium red onions, cut into thick wedges through root ends