I love the cheesiness of this gratin but if you’re vegan or just not that into cheese, omit it and the butter and use a good measure of olive oil to make the potatoes glisten. Chicken or vegetable stock can be substituted for cream. If you desire crunch, sprinkle with breadcrumbs tossed with olive oil towards the end of baking. Serves 8 to 10
1. Butter baking dish and grind black pepper to lightly cover bottom. Sprinkle 2 tablespoons of parmesan evenly in the bottom of the dish.
2. Mix the remaining grated cheese together in a small bowl. Reserve.
3. Thinly slice potatoes by hand or with a mandoline. Place in a bowl and toss with 2 teaspoons salt. Transfer to a strainer and let drain for 20 minutes.
4. Heat a large sauté pan over medium high heat. Add 2 tablespoons butter and heat until edges start to brown. Add the mushrooms and cook until they exude their juices.
5. Add shallots and thyme, season with salt and pepper and sauté until juices are almost dry. Adjust seasoning to taste, transfer to a bowl or plate and reserve.
6. Preheat the oven to 375 degrees F.
7. Squeeze the potatoes with your hands to remove excess water and transfer to another bowl. Toss with freshly ground black pepper and rosemary to evenly distribute.
8. Line the bottom of the baking dish and sides with 2 layers of potatoes. Overlap slices by one-third. Sprinkle with one-third of the mixed cheese. Top with half the mushrooms.
9. Add another 2 layers of potatoes. Top with one-third of cheese and remaining mushrooms. Layer the remaining potatoes on top but leave a small hole at the center so you can see the mushrooms. Pour the cream into the center hole slowly. Grind black pepper over.
10. Place in the preheated oven and bake for 50 minutes. Remove from oven, top with remaining cheese and place back in the oven for 20 minutes to bake until cheese is bubbly and golden. Serve warm or at room temperature.