Start with reducing the chicken stock by half.
Season the lamb leg with a drop of oil, pepper, and salt and roast it in the pan till all the sides are golden brown. Pour off the excess fat and add 25 grams of butter, garlic, and rosemary. Roast for 3 minutes, add the reduced chicken stock, and cook together for 5 minutes.
Cover the pan with a lid and place in a preheated oven at 180°c for 2 hours. Turn the meat halfway and make sure the liquid doesn't evaporate completely. If it gets too dry add a bit of water.
Now you can prepare the cabbage by slicing it finely, boil in lightly salted water, and steam dry on a colander. Chop the shallot, sweat it in a little knob of butter and add the cabbage. season with salt to your taste.
After 2 hours, remove the lid of the Lamb leg and roast for 20-30 minutes more. This is a critical time for your sauce so you have to keep a good eye on it now. You want the liquid to reduce to a beautiful sticky jus but not turn dry and too salty. Keep basking the meat with the jus till a shiny glaze is achieved and your sauce is rich and sticky. To thick? add some water. Not sticky? keep basking in the oven till the sauce is more reduced.
Finish the dish with the warm cabbage, glazed lamb leg, and serve the sticky jus on the side.