Vegetarian French Onion Soup with Mushrooms

Name: Vegetarian French Onion Soup with Mushrooms
Ingredients: ¾ stick unsalted butter; 2 large onions, peeled, halved, and thinly sliced (about 4 cups); 2 bunches scallions, white and light-green parts coarsely chopped (about ¾ cup), dark-green tops thinly sliced; 4 sprigs thyme, plus leaves for serving; 1 dried bay leaf; 12 ounces cremini mushrooms, coarsely chopped; Coarse salt and freshly ground pepper; 4 1 ¼-inch-thick slices sourdough bread; 1 ½ cups shredded Gruyere (about 4 ounces)