2. Coarsely chop two of the onions; add to the pot, along with the celery, carrots, bay leaves and thyme. Lower heat to medium and cook, stirring occasionally, until vegetables are soft and beginning to caramelize, about 10 minutes. Return the beef to the pot. Pour in 8 cups water. Simmer mixture gently until the meat is very tender, 2 1/2 to 3 hours.

3. Transfer beef to a bowl to cool for another use. Strain liquid into a bowl over a fine-mesh sieve; press gently on the solids with the back of a spatula to extract as much flavor as possible. Discard the solids; you should have about 10 cups broth (add water if necessary to equal 10 cups).

4. Halve the remaining 6 onions through the root end, then peel and thinly slice them lengthwise. Melt the butter in the bottom of the Dutch oven over medium heat. Add the onions and cook, tossing occasionally, until deep golden-brown and caramelized, 45 minutes to 1 hour. Season with 1 teaspoon salt and black pepper. Pour in the port and cook, scraping up any browned bits from the bottom of the pan, for 3 minutes. Pour in the broth and simmer mixture over low heat for 30 minutes. Season with salt and lemon juice, if desired. (For a smaller group, you could refrigerate some of the soup and reheat it later.)