Step 1
Whisk 1 garlic clove, finely grated, ¼ cup fresh lemon juice, 1 tsp. Dijon mustard, ½ cup extra-virgin olive oil, 3 Tbsp. buttermilk powder, ½ tsp. sugar, and ¼ tsp. Diamond Crystal or Morton kosher salt in a small bowl; season with freshly ground pepper. Set dressing aside.
Step 2
Toss 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, and 1 cup chopped dill and/or thinly sliced chives in a large bowl; set salad aside.
Step 3
Working with 1 of 3 medium skinless, boneless chicken breasts (about 1½ lb.), at a time, hold a long knife parallel to cutting board and cut in half, slicing along a longer side to divide into 2 thin cutlets. (They should be about ¼” thick. If too thick, cover with plastic wrap and pound with a meat mallet or the back of a cast-iron skillet.)
Step 4
Mix 2½ tsp. garlic powder, 2 tsp. onion powder, 1 tsp. freshly ground pepper, and remaining ½ cup buttermilk powder, 2 tsp. sugar, and 1½ tsp. Diamond Crystal or 1 tsp. Morton kosher salt in a shallow bowl. Dredge each cutlet in mixture to generously coat on both sides; arrange on a baking sheet. Sprinkle any remaining dredge over cutlets.
Step 5
Prepare a grill for medium heat. Place 2 medium red onions, cut into thick wedges through root ends, on a another baking sheet. Drizzle with oil, season with salt and pepper, and toss to coat. Grill onions, cut side down, until grill marks form and they release easily from grate, 1–2 minutes. Turn; grill until slightly softened, ends are lightly charred, and grill marks form on other side, 1–2 minutes. Return to baking sheet.
Step 6
Drizzle chicken with oil. Grill until dark grill marks appear and chicken
releases easily from grate (if stuck, use a spatula to help release), 1–2 minutes. Turn
and grill until cooked through, 1–2 minutes. Transfer to a plate; season with more salt
if needed.
Step 7
Add onions to reserved salad. Drizzle reserved dressing over, season with salt
and pepper, and toss to coat. Arrange on a platter and top with chicken. Drizzle any
dressing left in bowl over.