1. Cut the potatoes in half and set aside. Mix the remaining ingredients for the potatoes together. (Tip: oil helps prevent butter from burning on the grill.) Combine the mixture with the potatoes and toss to coat evenly.
2. Pour everything into a foil tray and cover with a piece of foil. Place the tray on the back of the barbecue at medium-high heat.
3. Cook the potatoes for 45 minutes, giving an occasional stir or shake, and start checking for doneness after 30 minutes. Serve with a dollop of horseradish sour cream.
Horseradish Sour Cream
1. For the Horseradish Sour Cream, mix all the ingredients together and let sit for at least one hour in the fridge before serving.