1 tablespoon butter

7 or 8 Spanish onions thinly sliced

Kosher salt

Freshly ground black pepper

6 to 12 slices of baguette or any country-style bread

1/3 cup sherry

Red or white wine vinegar (optional)

Red wine (optional)

1/2 to 3/4 pound Gruyère cheese, grated

Use a large pot, with a capacity of about 7 1/2 quarts, that will hold all the onions. An enameled cast-iron pot will provide the best surface. 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. 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). Season with several grinds of pepper.

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 indefinitely, as the heat is not high enough to burn them).

When the onions have completely cooked down, the water has cooked off, and the onions have turned amber—this will take several hours—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 some vinegar. If you would like a little more depth, add a splash of red wine. I like the onion-to-liquid ratio with 6 cups of water. But if you’d prefer a little more delicate soup, add 1 cup water.

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

Michael Ruhlman