- 2–3 pounds beef bones (preferably grassfed, see note*)
- 3 carrots, coarsely chopped
- 3 celery stalks, coarsely chopped
- 1 large yellow onion, coarsely chopped
- 2 bay leaves
- 5 garlic cloves
- 1 tablespoon fine sea salt, plus more to taste
- 2 tablespoons ghee or clarified butter
- 6 large yellow onions, sliced
- 10 cups beef stock
- 2 tablespoons flour
- ½ cup white wine
- 8 slices Gruyère cheese
- 8 slices crusty bread
1. Place a large sauté pan over medium-high heat. Add the beef bones and brown well on all sides, about 2 minutes per side. Place the bones in a slow cooker.
2. Deglaze the pan with ¼ cup hot water; use a wooden spoon to scrape off the parts that stick. Add this water to the slow cooker along with the carrots, celery, onion, bay leaves and garlic. Fill the slow cooker with water. Set the temperature to low and leave it overnight, about 12 hours.
3. In the morning, strain the broth through a large colander, discarding any solids. Turn off the heat and let the broth cool. The stock will become gelatinous, and the fat will rise to the top and harden. Skim the fat and taste the broth; season with salt to taste. Pour 10 cups of broth back into the slow cooker. Refrigerate the remaining broth in an airtight container for up to 4 days, or freeze for up to 6 months.
4. Melt the ghee in a large sauté pan over medium heat. Add the onions and sauté, stirring frequently, until browned, about 20 minutes. Add the flour to the onions. Cook, stirring with a wooden spoon, for about 1 minute. Deglaze the pan with the white wine. Add the 1 tablespoon salt and cook for 1 minute, then add the mixture to the slow cooker. With the slow cooker still on low, cook until the onions are very soft, about 8 hours.
5. Before serving, preheat oven to 400°. Arrange bread slices in a single layer on a baking sheet and bake until crisp, about 10 minutes. Divide the soup between individual ovenproof bowls. Float a slice of the bread in each bowl and top with a thick slice of Gruyère cheese. Set the oven to broil and melt until the cheese is golden brown, about 3 minutes. Serve hot.
*Note: You can pick up grassfed beef bones at a local farm or butcher shop.