traditional french onion soup

ingredients:

- 1 tbsp unsalted butter
- 7 or 8 lbs Spanish onions (about 10 to 15), thinly sliced
- Coarse salt
- Freshly ground black pepper
- 6 to 12 slices baguette or any other country-style bread, preferably large enough to cover the width of your serving bowls
- 1 litre beef broth
- Red- or white-wine vinegar (optional)
- Red wine (optional)

- 1/2 to 3/4 pound Gruyere or Emmenthaler cheese, grated

1. Place a 7 1/2-quart enameled cast-iron pot over medium heat; melt butter. Add onions and sprinkle with 2 teaspoons salt; cover, and cook until onions have heated through and started to steam. Uncover and reduce heat to low; cook, stirring occasionally, until amber and cooked down, about 3 hours. Season with pepper.

2. Meanwhile, preheat oven to 200F.

3. Place bread slices on a baking sheet and transfer to oven. Let dry completely, 20 to 30 minutes.

4. Add beef broth and enough water to pot with onions; increase heat to high and bring to a simmer. Immediately reduce heat to low. Season with salt and pepper. Add a splash of vinegar, if desired, to cut sweetness. For more depth of flavor add a splash of red wine, if desired.

5. Preheat broiler. Divide soup evenly among 4 to 6 ovenproof bowls; top with toasted bread. Divide cheese evenly among bowls. Transfer bowls to broiler; broil until cheese is melted and browned, 3 to 5 minutes. Serve immediately.

ref: https://www.marthastewart.com/867269/traditional-french-onion-soup