Preheat the oven to 350° F.Butter a 9x13 inch casserole dish. In a large glass measuring cup, combine the cream, milk, cider, garlic, thyme, onion powder and cayenne. Season well with salt, and pepper. Mix in the Gruyere cheese. Pour 1/3 of the milk mix into the bottom of the baking dish. Layer in 1/3 of the sweet potatoes. Add another 1/3 of the milk mix, then 1/3 of the potatoes. Add the remaining potatoes, then pour over the last of the remaining milk mix. Try to get the potatoes in an even layer.Cover and bake 1 hour 45 minutes to 2 hours. Remove the top, sprinkle over the parmesan cheese and rosemary. Bake another 20 minutes, until the cheese is golden and melty. Serve the dish warm, topped with fresh thyme.