I have recently made two very successful attempts at an oxtail soup.
1.2 kgs oxtail
Yellow onion about equal in volume, maybe slightly less
Carrots about 1/4 the onions, so 2 - 4 carrots depending on size.
I Brown the meat and precook the onions and the carrot.
My first attempt simmered for about 5 1/2 hours. (The "recipe" called for three) and when I got the tailbones out I had to use my hands a lot to get the meat off of the tails. Digging in good. I had to use a knife to get the gristle off.
My next attempt I added celery root and tomato puree in the first boil. After 3.5 hours I could easily get the gristle off by just thumbing it off. The meat came off the bone like pork cooked to well done + 5 hours.
They both tasted awesome, but behaved very differently. This does not seem to be a fire and forget dish.
Why did they come out so differently?
The soup is completed by using the broth from the first cooking, a good idea is to sift with a cloth. Add the meat, some veggies that you like, celery, mushrooms, carrot, season to taste and you have an amazing soup.