Heat the oil in a flameproof casserole and add the beef a few pieces at a time. Cook for a few minutes, then add the onion and fry over a high heat, stirring, until the meat is evenly browned.
Stir in the plain flour, then mix in the beer, 600 ml (1 pint) of the stock, the Worcestershire sauce, mustard and vegetables. Season and bring to the boil, stirring occasionally. Cover the dish and cook in a preheated oven, 160°C (325°F), Gas Mark 3, for 1½ hours or until the meat and vegetables are tender. Allow to cool then chill until required.
Mix the self-raising flour, suet, two-thirds of the cheese and a little seasoning in a bowl, cover and set aside.
When ready to serve, stir the mustard and enough water into the dumpling mix to make a soft smooth dough. Shape into 8 balls with floured hands. Bring the beef casserole back to the boil on the hob, adding the remaining stock, if needed. Stir well and add the dumplings, leaving space between them. Cover and simmer for 15 minutes until the dumplings are fluffy.
Sprinkle the dumplings with the remaining cheese then transfer the casserole to a preheated hot grill to melt and brown the cheese topping. Spoon into shallow bowls and serve immediately.