Step 1

Use a large pot (7 to 8 quarts) that will hold all the onions. Place the pot over medium and melt the butter. Add the onions, sprinkle with 2 teaspoons salt, cover, and cook until the onions have heated through and started to steam, about 30 minutes.

Step 2

Uncover, reduce the heat to low, and cook, stirring occasionally (you should be able to leave the onions alone for an hour at a stretch once they’ve released their water). Cook for about 3 hours at low to medium-low heat. Season with several grinds of pepper.

Step 3

Meanwhile, preheat the oven to 200°F. Place the bread slices in the oven and let dry completely (you can leave the slices in the oven for a few hours, as the heat is not high enough to burn them).

Step 4

When the onions have completely cooked down, the water has cooked off, and the onions have turned amber (3-4 hours total), add 6 cups of water. Raise the heat to high and bring the soup to a simmer, then reduce the heat to low. Add the sherry. Taste and season with salt and pepper as needed. If the soup is too sweet, add the vinegar. If you would like a little more depth, add a splash of red wine.

Step 5

Preheat the broiler. Portion the soup into bowls, top with bread, cover with cheese, and broil until the cheese is melted and nicely browned. Serve immediately.