Heat a grill to medium high heat.
Place the potatoes and onions in a foil container. Cut the butter into small pieces and scatter over the top.
Drizzle the potatoes with the water and Worcestershire sauce then sprinkle with the salt, pepper, garlic, and chili powder. Cover with a piece of foil and put on the grill.
Grill for 30-40 minutes, stirring twice and recovering, or until the potatoes are cooked through and the cheese is melted and bubbly.