Preheat oven to 350 degrees F.  Poke the sweet potatoes all over with a fork, slather them with a few tablespoons of butter, and roast until fork tender, about 40 minutes (cooking time varies with the size of the potato).  Remove the potatoes from the oven, allow them to cool, and then peel and mash.  Stir in the remaining butter and add the garlic, thyme, and cheese to your taste.  Place the sweet potato mixture in an ovenproof dish, grate a bit more cheese over the top, and bake until warmed through, about 30 minutes.  Leftovers freeze well.