American French Onion Soup Recipe

Ingredients for about 2 1/2 quarts of soup:
6 large yellow onions, cut in large dice
1/2 stick unsalted butter salt and pepper to taste
3-4 sprigs fresh thyme
1 or 2 tsps. sherry vinegar, or to taste
3 tbsps. dry sherry wine (do NOT use “cooking wine”)
4 cups high-quality beef broth
4 cups high-quality chicken broth
buttered croutons
shredded extra-sharp cheddar and Monterey Jack cheese (you’ll need about 1/3 cup per bowl)