About 15 minutes before the end of the cooking time add the garlic.
If the joint has a thick layer of fat cut this off. Rub joint all over with salt and pepper.
Heat oil in a saucepan over medium high heat. Sear beef on all sided. Pour in a little water. Brush mustard on top of the meat.
Reduce the temperature and simmer for 1 1/2 to 2 hours. Turn every now and then checking there is still some water in the dish. Add more if necessary. The meat should be sitting in the water, not swimming in it.
Remove the beef from the dish and keep warm. Pour the meat juices through a sieve and season with salt and pepper.