While the croutons bake, whisk the mayonnaise, buttermilk, sour cream, parsley, 1/2 teaspoon garlic powder, onion powder, dill, lemon juice, 1/4 teaspoon salt, and pepper in a bowl. It should be thick enough to coat the back of a spoon. Thin it with a teaspoon of buttermilk at a time if needed, then chill while you prep the salad.