Heat a dash of oil in a pan and fry the shallot and mushrooms until soft, then leave to cool.
Mix the fried vegetables with the mince, breadcrumbs and mustard.
Shape into two sausages the length of the beef slices, then roll the beef slice around the mixture. Tie with kitchen string, and season with salt and pepper.
Heat a little more oil in a pan and fry until golden-brown all over. Add the onions and cook until they have coloured a little, then add the flour and beer. Simmer for 5 minutes, then add the stock. Loosely cover and simmer for 30 minutes.
Meanwhile, toss the leeks and carrots in olive oil and roast in the oven for 25-30 minutes.