This one pan mushroom gratin with white beans is a hearty and flavorful dish that is perfect for a cozy dinner. The mushrooms are sautéed until golden brown and then combined with creamy white beans and a rich, cheesy sauce. Topped with a crispy breadcrumb topping, this gratin is baked until bubbly and golden. Serve it as a main course with a side salad or as a delicious side dish for roasted meats or poultry.