Creamy Mushroom-Potato Gratin

Ingredients (11)

• 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