The Kitchn: Casserole a perfect place for leftover onions, day-old bread
For the stew:
8 tablespoons (1 stick) unsalted butter, divided
7 large yellow onions (5 to 5 1/2 pounds total), halved and cut into 1/4-inch-thick slices
4 large shallots (about 8 ounces), peeled and cut into small dice
4 teaspoons kosher salt, divided
1 teaspoon freshly ground black pepper, divided
6 cloves garlic, minced
5 stalks celery (about 12 ounces), including leaves, cut into small dice
5 carrots (about 8 ounces), peeled and cut into small dice
6 cups cremini and/or button mushrooms (about 1 1/2 pounds), sliced thinly
...continued