Place the chicken breasts in a large ziplock bag and pour in the buttermilk. Marinate for 30 mins or overnight.
Preheat the oven to 375 F. Remove the chicken breasts from the bag and place them in a baking dish.
Season them with salt and pepper on both sides.
Make the topping by mixing the Greek yogurt (or mayo), half of the parmesan cheese, minced onion, garlic powder, and parsley in a small bowl.
Divide the mixture evenly over the chicken breasts and sprinkle the remaining Parmesan cheese on top. Bake for 40 minutes or until golden brown. If they're not brown when the cooking time is over, place them under the broiler for the last 5 minutes.