4 lbs. yellow onion, sliced lengthwise
3 Tbsp butter
1 tsp salt
1/2 cup dry sherry
4 cups chicken broth
2 cups beef broth
2 cups water
1 tsp thyme (several sprigs)
1 bay leaf
1/2 tsp salt
cracked black pepper and salt to taste
french baguette, 1/2 inch slices
1/4 lb. gruyere cheese, grated (about 1 1/2 cups)
Preheat the oven to 400 degrees. Peel the onions, halve them, and slice lengthwise. Spray the inside of a dutch oven with cooking spray then add the butter, onions, and 1 teaspoon of salt. Cover with the lid and place in the oven and cook for 1 hour.
Stir the onions after the one hour of cooking, being sure to scrape any browned bits away from the edges. Replace the lid, leaving it cracked open about 1/4 to 1/2 inch. Cook for 1 1/2 hours, stirring again after 1 hour.
Place french bread slices on a baking sheet in the 400 degree oven for 10 minutes. Move the dutch oven to the stove over Medium-High heat. Stir the onions often and watch carefully while cooking for 15-20 minutes while a fond develops. Add 1/4 cup of water to deglaze the pan and cook until water evaporates and another fond forms, about 5-7 minutes. Repeat this process 2-3 times until a rich, dark mahogany color develops in the onions. Add 1/2 cup dry sherry and cook off the alcohol for about 5 minutes.
Add the 4 cups chicken broth, 2 cups beef broth, 2 cups of water, 1/2 tsp salt, thyme, and bay leaf to the pan. Bring to a boil, then reduce heat to low and simmer for 30 minutes with the lid on. Check seasoning, adding salt and pepper to taste.
Ladle soup into 6-8 oven safe bowls, float french bread croutons in each bowl, and top with shredded gruyere (about 1/4 cup for each bowl). Place under the broiler until cheese bubbles and begins to brown (about 2-3 minutes).