-
1
Heat the oven to 400 degrees.
-
2
Place the onions and butter in large lidded casserole, season with one-fourth teaspoon salt, cover and place in the oven.
-
3
Cook, stirring every 15 minutes or so, until the onions are softened and a light golden-brown, about 1 1/2 hours.
-
4
Transfer the casserole to the stove-top and heat over medium-high heat.
-
5
Continue to cook, stirring frequently, until the onions are a deep golden-brown and just begin to stick to the bottom of the pot to form a crust.
-
6
Add half of the water and cook, scraping the bottom constantly, until the water has evaporated, about 8 minutes.
-
7
Add the Sherry and continue cooking until it has evaporated, 3 to 5 minutes.
-
8
Stir in the remaining water, chicken broth and beef broth, as well as the thyme bundle and bring to a gentle simmer.
-
9
Reduce the heat to maintain a bare simmer and continue to cook for 40 minutes.
-
10
Remove the thyme and season to taste with salt and pepper.
-
11
While the soup is simmering, prepare the toasts: Slice the French bread on a bias into one-half-inch thick slices large enough to fit inside of each soup bowl (use one toast if it will fit, two if the slices are smaller).
-
12
Toast the bread until crisp and golden.
-
13
Set aside.
-
14
Fill 8 oven-proof soup bowls with soup to one-half inch from the rim, making sure to distribute onions evenly between bowls.
-
15
Lay the toasts on top of the soup, inside the edges of the bowl.
-
16
Sprinkle the grated Gruyere evenly over the tops of the bowls and place the bowls under the broiler just until the cheese is bubbling and begins to brown in places.
-
17
Serve immediately.
