Preheat BGE to 400. While egg preheats, cut the top off a head of garlic, sprinkle with salt, drizzle with some olive oil, fold up in some foil, and put into the egg to roast. Also, make sure you soak 4 or more cedar planks.
While BGE preheats and garlic cooks, peel the potatoes and yam.
Put the potatoes in a pot with salted water. Cook till tender.
Remove the garlic from the egg, and smash 2 cloves. Add 1/2 the cheese, the butter, the cream, the sour cream, the roasted garlic, and salt and pepper to taste.
Mince the herbs.
Assemble remaining ingredients.
Rice the potatoes.
Dry off the planks and smear with butter.
Pile each plank with a serving of the potato mixture and place in the egg.
Leave on BGE for about 30 minutes or until the light crust has set.
Remove and sprinkle with remaining cheese. Sprinkle with the chives and parsley.
Serve on the plank or use a pastry spatula to slice onto a plate.