INGREDIENTS

• 3 tablespoons unsalted butter
• 1 small yellow onion, finely chopped
• 1 teaspoon kosher salt
• 1 pound cremini mushrooms, sliced
• 2 teaspoons minced fresh thyme
• 1⁄4 teaspoon freshly ground black pepper
• 2.5 pounds russet potatoes, peeled and sliced 1/8-inch thick
• 1 cup low-sodium chicken broth
• 1 cup heavy cream
• 2 bay leaves
• 2 packed cups shredded Gruyère, Fontina, or white cheddar cheese

1. Preheat the oven to 400°F. In a stockpot or large saucepan over medium-high heat, melt the butter. Add the onion and salt and cook, stirring occasionally, until softened, about 4 minutes. Add the mushrooms, thyme, and pepper and cook until the mushrooms are tender, about 5 minutes. Scrape into a bowl.
2. Put the potatoes, broth, cream, and bay leaves into the stockpot and bring to a simmer. Cover, reduce the heat to medium-low, and simmer until the potatoes are almost tender, about 10 minutes. Remove the bay leaves.
3. Transfer the mixture to a 1 1/2-qt baking dish. Sprinkle with the cheese. Bake until bubbling and golden brown, about 20 minutes. Let cool for about 5–10 minutes before serving.

https://www.chowhound.com/recipes/creamy-mushroom-potato-gratin-31529