Heat the oil in a pan, season the meat and then brown on both sides. Take your time to get a good sear since it will add depth and flavor to the final sauce. Add the onions and garlic and cook another few minutes.
Transfer all to the slow cooker and add remaining ingredients for the braise. Set the cooker for 4–6 hours, checking occasionally to make sure there is enough liquid to come halfway up the meat, adding a little water if necessary.
When the meat is falling-apart tender, remove from the cooker. Shred the meat with a fork. Return the remaining braising sauce to the stove, simmer until thick and add it back to the meat.
While the beef is braising, toss together all the ingredients of the kohlrabi slaw and put it in the fridge to let the flavors meld.
When ready to eat, put a large spoonful of beef on a bun, top with the slaw and serve.