3 large sweet white onions, thinly sliced
8 cups beef broth (homemade is best or use a good quality beef cooking stock)
1 stick butter (not margarine)
1 teaspoon sugar
Salt and pepper to taste
3 tablespoons all-purpose flour
1/3 cup dry vermouth
French bread
Grated Gruyere cheese

In a crockpot, add the beef broth, and turn the crockpot to low.

In a large skillet, melt the butter over medium heat. Add the thinly sliced onions, and cook slowly to caramelize the onions, stirring occasionally. You want to develop the flavors and sweetness of the onions, but you don’t want them to overbrown.

When the onions are nicely caramelized, add salt and pepper to taste and the sugar to the onions. Stir until well combined. Add the flour to the onion mixture, and cook until the flour is nicely browned (as if making a roux). 

Add the dry vermouth, and stir until the liquid is evaporated. 

Add the onion mixture to the beef broth in the crockpot, scraping everything from the pan. Cook on low for 4 to 6 hours.

When ready to serve, preheat the oven to 450 degrees Fahrenheit. Ladle the soup into ovenproof individual serving crocks. Place a slice of French bread on top of the soup, and cover the top of the bread with the grated Gruyere cheese. Place the soup crocks into the oven, and bake for approximately 7 minutes, or until the cheese is browned and bubbly.