Prepare the carrots, onions, celery, and leeks and toss 71 together, reserving half the mix. One half is for your stock and the other for the soup. In a large stockpot big enough to hold all the ingredients, place the bone marrow standing up on the bottom and cook on medium heat for about 3 minutes. Flip the marrow over and cook another 3 minutes. Add all the vegetables and the4 quarts of the beef bone broth.
Bring to a boil then simmer for 3 hours. The longer you simmer it the better it is, so if time permits, cook it for another hour. Add more water or broth as needed as the liquid will reduce. When your broth is done, remove the bones carefully and place them in an oven-proof glass baking dish big enough to hold the bones. Place in a 250-degree preheated oven. Strain the broth very carefully into another stockpot through a fine sieve, discarding the vegetables. Add the other half of the prepared vegetables into the pot.
Cook on medium heat for 30 minutes. Season with salt and black pepper to your taste. I recommend a lot of black pepper as it’s an anti-inflammatory. To serve, ladle the broth and vegetables into a large soup bowl, add one or two marrow per person, depending on if you’re serving 4 or 8 people. Serve with a mini fork to pull the marrow out. Spread the marrow on the crispy warm sourdough toast, if desired. It is a treat you will never believe. *Note- you will need mini forks to remove the marrow.