1 tablespoon unsalted butter
1 teaspoon extra-virgin olive oil
1 1/2 pounds yellow onions, peeled, halved, and thinly sliced, (about 6 cups sliced)
6 shallots, thinly sliced, (1 3/4 cups)
1 leek, white and light green part thinly sliced, washed well, and dark green top reserved
Kosher salt and freshly ground black pepper
3 to 4 fresh thyme sprigs
1 fresh rosemary sprig
1 tablespoon all-purpose flour
4 cups low-sodium beef broth
2 cups water
2 teaspoons Worcestershire sauce
1/4 cup dry sherry
8 (1/4-inch thick) slices whole-wheat baguette (2 ounces), toasted
1/2 cup finely grated Gruyere (about 1 3/4 ounces)
1 tablespoon chopped fresh chives