1 stick (4 ounces) unsalted butter
6 cups thinly sliced Vidalia or Spanish onions
Kosher salt and freshly cracked black pepper
1 tablespoon all-purpose flour
1/4 cup dry sherry
5 cups beef stock
6 sprigs fresh thyme
1 cup grated Gruyere cheese
12 thin slices baguette
In a large pot, melt the butter over medium heat.
Add the onions all at once and a small splash of water. Season the onions with salt and pepper.
Bring the onions to a simmer, stirring often. Reduce the heat if the onions begin to cook too fast.
Cook until the onions are soft and translucent, about 15 minutes. (If you want a sweeter soup, continue to cook the onions until they caramelize to the richness of your preference.)
Add the flour and cook for 3 minutes. Add the sherry and cook for a few minutes longer. Add the beef stock and bring to a very low simmer; simmer for about 35 minutes.
Preheat the broiler to finish the soup.
Add the thyme to the soup and simmer for 5 more minutes. Season with salt and pepper as needed. Remove the herbs.
Toast or grill the baguette slices.
Ladle the soup into 4 soup crocks. Cover some baguette slices, then top with the shredded Gruyere. Transfer the crocks to a baking sheet and place under the broiler until the cheese bubbles and browns.