In a small sauce pan, heat cream with garlic and thyme until it is heated through, but not boiling. Season with salt and pepper.
1 ½ cups heavy cream, 1 teaspoon thyme, 1 teaspoon minced fresh garlic, salt and pepper
Peel potatoes, cut into ⅛" slices and place in the baking dish.
5 medium russet potatoes,
Pour the warm cream sauce over the potatoes. Sprinkle the shredded cheeses on top and add the mushrooms. Sprinkle with parmesan cheese.
Bake for 45 minutes or until browned and bubbly. Let gratin sit for 10 minutes before serving.
Notes
Expert Tips✘ Use the right potato. Stick with russet potatoes for this recipe as they do the best job of keeping their structure when baked in cream. Don't use Yukon Gold potatoes! Their water content is too high and the end result is a soupy mess.✘ Heavy cream is a must. Ok, not really a must but to get the best potato and mushroom gratin you can, the heavy cream definitely makes a difference.✘ Heat the cream, garlic, and thyme. This step seems like it would be a no-brainer to skip, but the fragrant flavor that cream gets when it is infused with garlic and thyme is a game changer.Make-Ahead TipsTo Prep Ahead of Time:Follow these steps to ensure your potatoes do not turn gray. 1. After you peel the potatoes, place them in a bowl of cold water when you are waiting to slice them. 2. As soon as they are sliced, place them in the baking dish and pour the cream over the top, making sure to coat each potato slice. 3. Cover them with shredded cheese and mushrooms. When potatoes are exposed to air they will turn gray. Covering the top of the dish with the cheese and potatoes will prevent this. 4. Wrap them tightly with plastic wrap pressing down on top of dish to keep all air out. Refrigerate until ready to bake.
To Bake Ahead of Time:
Simply follow the recipe as written and let cool completely before wrapping the baked Potato Gratin with foil and refrigerating.
When you are ready to reheat the gratin, remove it from the refrigerator an hour before heating to allow it to come to room temperature.
Reheat in a preheated 350°F oven for 30 minutes or until warmed through