Potato, Leek, and Mushroom Gratin

Ingredients

• 0.1 lb butter 
• 0.3 cup cooking wine dry white
• 1 pound mushroom caps fresh rinsed sliced
• 1 tablespoon thyme leaves fresh minced
• 1 tablespoon garlic minced
• 12 oz gruyère cheese shredded
• 4  leeks ()
• 1.8 cups chicken broth low-sodium
• 0.5 teaspoon pepper 
• 3.8 pounds russet potatoes 
• 1 teaspoon salt 
• 1.8 cups whipping cream 

Equipment

• bowl
• frying pan
• oven
• whisk
• baking pan
• aluminum foil

Directions

1. In a 12-inch frying pan or 5- to 6-quart pan over medium-high heat, melt 2 tablespoons of the butter.
2. Add mushrooms and stir often until almost tender, 4 to 5 minutes.
3. Add garlic and stir until mushrooms are golden and tender to bite, 2 to 3 minutes longer.
4. Transfer to a bowl.
5. Trim and discard root ends and tough green tops from leeks.
6. Cut stalks in half lengthwise and rinse well under running water, flipping layers to release dirt.
7. Cut crosswise into 1/4-inch slices; you should have about 4 cups.
8. In the frying pan (no need to wash), over medium-high heat, melt remaining 2 tablespoons butter.
9. Add leeks and thyme and stir often until leeks are limp and beginning to brown, 8 to 10 minutes. Stir into mushrooms.
10. Preheat oven to 35
11. Peel and slice potatoes crosswise as thinly as possible (see notes). In a bowl, whisk together broth, cream, wine, salt, and pepper. In a buttered 9- by 13-inch baking dish (3-qt. capacity), evenly layer a third of the potatoes, half of the mushroom-leek mixture, and 1 cup cheese. Stir broth-cream mixture and pour a third evenly over the top. Repeat.
12. Layer remaining potatoes, cheese, then cream mixture on top. Set dish in a 10- by 15-inch baking pan.
13. Bake until potatoes are tender when pierced and sauce is bubbly, 1 1/2 to 1 3/4 hours. After about 40 minutes, check browning; if top begins to get dark before potatoes are tender, cover loosely with foil.
14. Let stand about 15 minutes before serving.