Recipe: French Onion Soup Casserole

Name: Recipe: French Onion Soup Casserole
Ingredients: 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; Cooking spray; 1/4 cup all-purpose flour; 1 cup dry Riesling wine; 6 to 8 fresh sage leaves, minced, or 2 teaspoons dried sage; Leaves of 5 sprigs fresh thyme, finely chopped; 1 (15-ounce) can white beans and their liquid (optional); 1 large baguette (about 1 pound), cut crosswise into 24 slices; 12 ounces Gruyere, Comté, or Raclette cheese, shredded; 4 tablespoons unsalted butter