6 large yellow onions, diced
1/4 cup unsalted Land O Lakes butter
3 tablespoons dry sherry, optional
1 tablespoon sherry vinegar
4 cups chicken broth
4 cups beef broth
2 sprigs fresh thyme
salt and ground black pepper to taste
6 slices French bread
1/4 cup Land O Lakes butter, melted
1 cup shredded Cheddar cheese (extra sharp)
1 cup shredded Monterey Jack cheese
Preheat oven to 425F.
In a large, oven-safe skillet, melt 1/4 cup butter over medium heat. Add the diced onions and stir until well-coated in butter. Transfer skillet to the oven, cook onions until well-browned and tender, for an hour. Stir occasionally.
Transfer skillet back to the stovetop. Keep cooking and stirring the onions (medium heat) until they start to stick to the bottom of the pan. This should take about 5 minutes.
Add sherry (if not using, add a bit of water instead) and vinegar, bring to a boil and scrape the browned bits off the bottom of the pan, using a wooden spatula.
Place the onions in a soup pot. Add the chicken broth, thyme and beef broth. Bring to a boil and skim off foam and fat that appears on top. Simmer on low heat for approximately an hour. Season with salt and pepper.
Preheat the broiler, set the oven rack to the top part of the oven (6″ from the heat source).
Brush the bread slices with melted butter (don’t skimp!) Place the slices on a baking sheet, and broil for a few minutes until golden brown.
Ladle the soup into oven-proof soup bowls, add a toasted slice of bread on top of each serving, sprinkle with cheddar and Monterey Jack. Broil for a few minutes, until cheese turns golden and bubbly.