Rub chicken breasts with olive oil, garlic powder, onion powder, paprika, and coarse salt.
Heat grill or grill pan over medium-high heat and cook chicken for 5 to 7 minutes per side until fully cooked. Let rest.
Melt butter in a small pan over medium heat; add minced garlic and cook for 30 seconds.
Stir in pepperoncini brine and chopped peppers, then simmer for 2 to 3 minutes until slightly reduced.
Combine halved tomatoes, chopped cucumber, and sliced red onion in a bowl.
Toss salad with olive oil, vinegar, salt, and black pepper.
Serve grilled chicken topped with pepperoncini garlic butter alongside cucumber salad and optional sides such as feta, hummus, or pita bread.