2 cups milk
1 cup whipping cream
2 cloves garlic, minced
2 tablespoons fresh thyme, minced
2 tablespoons olive oil
3 pounds mushrooms, sliced
1 leek, cut into rings
Salt and black pepper to taste
3 pounds baking potatoes, thinly sliced
3 tablespoons Italian parsley, chopped
3 tablespoons chives, chopped (optional)
1-1/2 cups fresh Parmesan or Asiago cheese, grated
Preheat oven to 375 degrees. In a saucepan, combine milk, whipping cream, garlic and 1 tablespoon of thyme over low-to-medium heat. Heat until just under a boil. Remove from heat, cover and let set while preparing remainder of dish.
Coat a skillet with olive oil and place over medium heat. Add remaining thyme. Sauté the mushrooms and leeks for 5 minutes to evaporate the liquid. Season with salt and black pepper to taste.
Butter a large casserole dish. Arrange a third of the sliced potatoes on the bottom of the pan, overlapping them slightly. Next, season the potatoes with parsley, chives, salt and black pepper.
Top potatoes with half of the mushroom mixture. Repeat, layering a third of the potatoes and remaining mushroom mixture. Arrange remaining potatoes on top. Press the layers down with a spatula to condense. Pour the milk over the potatoes. Sprinkle the surface with cheese. Cover the pan with foil and place in the oven. Bake for one hour or until the potatoes are tender. Uncover and bake for 15 more minutes until cheese melts and gratin is golden around the edges. Let stand for 10 minutes before cutting.