- This recipe calls for three pounds of yellow onions and 10 cups of beef broth. Sweet onions are marvelous here if you can get them. Feel free to use a combination of yellow and red onions, if needed.
- The onions cook alone for 12 hours. Opt for cooking overnight if you plan to serve the soup for dinner the next day. After adding the broth and seasoning to the soup, cook agin for another six to eight hours. The total cook time for this recipe is about 20 hours in the slow cooker.
- You will smell this soup cooking. But it smells delicious, so it’s OK!
- Slice the onions into quarter-moons. You can use your mandoline, a food processor, or a chef’s knife to make quick work of this.
- Cook the onions with butter and salt on low for 12 hours: This step is going to perfume the whole house. A reader suggested doing this step on a screened-in porch or in your garage if you need to avoid your home smelling like cooking onions for a whole day.
- Add the broth and vinegar and cook for six to eight more hours on low: Again this can be done without supervision; there’s no stirring or checking.
- Top with toasted bread and Gruyère and bake before serving: This final step really helps to thicken the soup. The baking allows for more evaporation and some of the starch from the bread softens and thickens the soup as well.
Cook for LOW 6 to 8 hours. Cover and continue cooking on the LOW setting for 6 to 8 hours. This is flexible; as long as your slow cooker holds moisture well (wrap a towel over the lid if quite a lot of steam escapes), you can cook the soup for hours. Longer cooking will only intensify the flavors. Taste and season with more salt and pepper if needed, and stir in the brandy if using.
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