Put raw sliced onions in casserole. 3 ingredients later: Comfort food you didn’t know you needed

For a vegetarian version, substitute the beef broth with vegetable broth. You can also add a layer of cooked mushrooms for an earthy flavor. If Gruyère is not available, Swiss cheese or mozzarella can be used as alternatives. For a bit of a kick, consider adding a pinch of red pepper flakes to the onions while they caramelize. To enhance the dish’s texture, mix some chopped nuts, like walnuts or pecans, into the breadcrumbs before topping the bake.