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Remove it from the fridge about 1 hour before you want to heat it. Reheat covered in foil in a 350º F oven for 20 to 30 mins ...

Finish by topping with remaining potato mixture (making sure to add all the cream), Gruyère, and Parmesan; reserve remaining 1/2 tsp. thyme. Bake potato gratin until golden brown on top and bubbling, ...

The milk should become very thick. Next, whisk in the gruyere cheese, season with salt and pepper and in the herbs. Mix in the sliced potatoes to the pot and transfer to a small baking dish.