If using beef tendon, boil for at least 2 hours on its own before adding to below (or use a pressure cooker).
Add water and stock to large pot and bring to boil - add short ribs. Some people blanch the short ribs and then throw out the water and start again (for a clearer stock), but it's not essential.
Lower heat and simmer meat (with tendons, if using), for 3 hours. (Using a pressure cooker will shorten all of this significantly...but I don't have one yet so if anybody tries it out, please comment below ;)
Once beef has gotten tender, add ginger and radish. Cook for another 20 minutes.
Add japchae noodles and cook for 5 minutes.
Then add onion, scallions, garlic, and seasonings, and cook for another 5-10 minutes.
Serve with kimchi or eat on its own! This is a hearty, filling soup...good for winter (or, you know, hungry time) :)
Recipe by J.S. Hunter Fiction at https://jshunterfiction.com/kalbi-tang-korean-short-rib-soup/