This is really a template more than a recipe....
Prepare bone broth -
Roast oxtails (weight depends on how much stock you want) in oven at 200C for 30 minutes. Place in heavy-bottomed pot or slow cooker, cover with water, add 1/4-1/2 cup apple cider vinegar or lemon juice. You can also add vegetables to your broth sich as onions, carrots, cellery, etc. Bring to a boil, then reduce to simmer for as long as you generally do for bone broth (24 hours is good).
Once bone broth is finished, let cool enough to handle then strain the liquid through a colander. Discard any vegetables used in the broth and remove the meat from the oxtails - set aside to use in the soup. You can compost the leftover bits.
To make the soup -
Saute onions, garlic and carrot in olive oil until soft. Add tomato paste, bone broth, diced veggies of choice, shredded greens and seasoning.
Cook until veggies are cooked through and serve hot.
Use leftover broth within 5 days or freeze for future use.
Enjoy - N x