Preheat a grill or griddle pan to medium-high heat.
Season both sides of the chicken with Italian seasoning, parsley, garlic powder, chili flakes, salt, and pepper; rub with the olive oil.
Place the chicken on the preheated pan and cook 7–8 minutes per side, or until golden and cooked through.
Transfer the chicken to a plate and let it rest at least 5 minutes, then slice or cube.
Meanwhile, whisk together the Greek yogurt (or sour cream), crushed garlic, and lemon or lime juice in a bowl; season with salt and pepper to taste.
Arrange the chopped romaine in a large bowl or divide between plates, then top with sliced chicken, sliced avocado, and halved grape tomatoes.
Drizzle the creamy dressing over the salad and serve immediately.