Potato Gratin with Mushrooms and Gruyère

Ingredients

• 0.5 teaspoon pepper black freshly ground ()
• 2  garlic clove minced
• 1 cup gruyere cheese grated
• 1.5 pounds seasoning cubes assorted stemmed (such as crimini and shiitake; 10 cups)
• 2 cups cup heavy whipping cream 
• 4 cups leek white green finely chopped ( and pale parts only; 3 large)
• 0.3 cup olive oil 
• 1 teaspoon salt ()
• 3 pounds yukon gold potatoes peeled cut into 1/8-inch-thick slices

Equipment

• frying pan
• oven
• pot
• aluminum foil
• kitchen towels

Directions

1. Heat 1/4 cup oil in large skillet over medium-highheat.
2. Add leeks; sauté until soft andlightly browned, 10 to 12 minutes.
3. Addmushrooms, sprinkle with salt and pepper,and sauté until soft and liquid evaporates,7 to 8 minutes.
4. Add garlic; sauté 1 minute.Season with salt and pepper. Set aside.DO AHEAD: Can be made 4 hours ahead.
5. Letstand at room temperature.
6. Preheat oven to 375°F. Pat potatoslices dry with kitchen towel.
7. Combinecream, 1 teaspoon salt, and 1/2 teaspoonpepper in large pot.
8. Add potatoes. Bring toboil; reduce heat to medium and simmer,covered, 10 minutes, stirring occasionally.
9. Remove lid; simmer until cream is reducedby about half and potatoes are partiallycooked, stirring often and watching closelyto prevent mixture from burning, about3 minutes. Season with salt and pepper.
10. Brush 13 x 9 x 2-inch glass or ceramicbaking dish with oil.
11. Transfer half of potatomixture to dish, spreading out in even layer.Spoon mushroom mixture over in evenlayer. Spoon remaining potato mixture over,spreading in even layer.
12. Sprinkle cheeseover. Cover with foil, tenting in center toprevent cheese from sticking to foil.
13. Bake30 minutes. Uncover; bake until potatoes aretender and top is brown, 20 to 25 minuteslonger.
14. Let rest 10 minutes before serving.