2 pounds sweet white onions (such as Vidalia), halved and thinly sliced (2 large onions)
¾ pound shallots, thinly sliced (8 to 10 small shallots)
2 thyme sprigs
1 garlic clove, smashed
1 bay leaf
3 tablespoons unsalted butter
¾ teaspoon kosher salt, plus more to taste
8 cups low-sodium beef broth
Freshly ground black pepper, to taste
1 large baguette, cut into ½-inch-thick slices
8 ounces Gruyère cheese, thinly sliced or grated