Accordion potatoes aka Hasselback potatoes may be even more delicious when they are grilled than when they are roasted. You be the judge.
Make the sauce. In a pot on medium heat, or direct heat of the grill add the heavy cream, sage, a nice pinch of salt, and a pinch of paprika. Bring to a simmer and steep for 10 minutes. Strain the heavy cream and discard the sage. Set aside.2.
Prepare the potatoes. Brush the bottom and sides of a large 9-10" cast iron pan with the melted butter and set aside the remaining butter. Arrange potato slices upright in a spiral around the dish (not flat like a traditional gratin). Between every 8-10 slices tuck in some garlic and randomly tuck in the onion slices as well. Sprinkle the chili flakes over the top. Add the sauce, filling to just below ¼-½" of the rim. Drizzle the leftover butter. Cover tightly with aluminum foil and grill over indirect heat until a fork pokes into the potatoes easily, about 20-25 minutes.3.
Uncover and finish. Remove the foil and top with cheese. Close the grill to allow the cheese to melt for another 5 minutes. Garnish with parsley and serve.