Heat half the vegetable oil in a large casserole dish or frying pan over medium heat for the first batch of lamb.
Cook lamb in 2 batches for about 5-10 minutes or until browned. Add remaining oil before cooking the second batch of lamb. Set aside on a plate.
Add 1/2 cup chicken stock to the pan and using a wooden spoon scrape the browned bit from the bottom of the pan.
Add onion, carrot & celery and cook, while stirring regularly, for about 5 minutes or until stock has evaporated and onion has softened.
Add mustard, worcestershire sauce, beer and remaining stock to the casserole dish and stir to mix through. Bring to a boil.
Return lamb to pan with potatoes and stir to mix through.
Reduce heat to simmer, cover casserole dish and cook for about 1 hour, or until lamb and potatoes are tender. Check potatoes with a fork or skewer to make sure they are cooked.