Make the stew: Heat 4 tablespoons of the butter in a large, heavy-bottomed pot or Dutch oven over medium-high heat until melted and just beginning to foam.

Add the onions, shallots, 1 teaspoon of the salt, and the pepper. Cook, stirring occasionally, until the onions are translucent and begin to brown on the edges, 5 to 7 minutes. Reduce the heat to low (or the lowest your stovetop will go) and cook, stirring occasionally and adjusting the heat as needed, until the onions are completely softened and caramel brown in color, about 30 minutes.

Add the garlic, celery, carrots and mushrooms (add the mushrooms in batches if they do not all not fit into your pot at once, as they will cook down and reduce in volume), and stir to combine. Once you have added all the mushrooms, cover and cook 20 minutes more.

Arrange a rack in the middle of the oven and heat to 375 F. Coat a 9-by-13-inch baking dish with cooking spray and set aside.

When the vegetables are ready, transfer them with a slotted spoon to a large bowl and set aside. Pour the remaining vegetable cooking liquid into another bowl or a measuring cup with a spout; set aside.

Return the pot to high heat, add the remaining 4 tablespoons of butter, and cook until it is melted and begins to foam. Add the flour, whisk to combine, and cook until it is golden-blonde in color, about 2 minutes. Add the wine and whisk to combine.

Add the reserved vegetable cooking liquid and whisk to combine. Add the sage, thyme and remaining 3 teaspoons salt and whisk to combine. Add the reserved cooked vegetables and the beans and their liquid, if using, and stir to combine. Pour the mixture into the casserole dish and spread into an even layer; set aside.

Make the crouton topping: Arrange 12 of the bread slices on top of the casserole in a single layer, pushing them down gently into the liquid and vegetables. Sprinkle with about half of the cheese. Arrange the remaining 12 slices of bread over the cheese in a single layer, then sprinkle the remaining cheese on top. Melt the butter in the microwave, then drizzle over the casserole.

Bake until the cheese is melted and the casserole is bubbly, 20 to 22 minutes. Serve immediately.

Recipe notes: This recipe can be broken into parts and cooked over several days: Make the onions one day, cook it with the vegetables the next, then make the sauce and put the casserole together on the final day. Reheat the vegetable mixture before finishing the casserole.

You can also make this recipe ahead a second way, if you are short on time. Prepare the onions as written, but then transfer them and all the vegetables to a slow cooker, stir, and set at low or medium-low and cook for 3 to 4 hours. Then scoop the vegetables out, make the roux (flour and butter mixture) on the stove, add the wine and vegetable cooking juices to the pan to make the sauce, and add the cooked vegetables as written.

This dish is best eaten fresh, as the bread absorbs liquid as it sits and the casserole will get soggy. If you have any leftovers, they can be stored in an airtight container in the refrigerator for up to three days.

(Tami Weiser is a contributor to TheKitchn.com, a nationally known blog for people who love food and home cooking. Submit any comments or questions to editorial@thekitchn.com.)