6 cups thinly sliced sweet onions (basic yellow or Vidalia)
1 TBS all-purpose flour
1/3 cup dry sherry (like from your local liquor store, not that "cooking wine" crap found on the vinegar aisle)
5 cups beef broth
6 springs fresh thyme, tied into a bundle with food-safe kitchen twine.
2 cups grated Gruyere cheese
12-16 1/4-inch thick baguette slices (basically you want enough bread slices to cover the top of your soup bowls)
Kosher salt and pepper
Olive oil spray, non stick spray, or other method to toast your bread
Procedure
In a Dutch oven, melt the butter over medium-low heat. Add the onions and a 1 TBS of water, and season them with about 1/2 tsp. of salt. Cook the onions, stirring occasionally until they caramelize, about 30-45 minutes. If the onions cook too fast, lower the heat so they don't burn.
Add the flour and stir to coat the onions. Cook for 3 to 4 minutes, then add the sherry. Cook 1 to 2 minutes, then add the beef broth and thyme bundle. Bring to a low simmer, and cook 40 minutes, stirring occasionally.
While the soup is cooking, spray your baguette slices on both sides with olive oil spray and season with a sprinkle of kosher salt and pepper. Toast the slices on both sides in a skillet over medium high heat.
Preheat your broiler. Place your 4 soup bowls on a rimmed baking sheet. Remove the thyme bundle from the soup carefully, and test the soup for seasoning, adding salt and pepper if desired.
Ladle the soup into the bowls, and arrange toasted baguette slices on top for full coverage. Sprinkle cheese evenly over each bowl (about 1/2 cup per bowl).
Broil the soup until the cheese is brown and bubbly to your liking. Enjoy!