Beef Stroganov With Mustard

1 kg fillet of beef
Salt and pepper
30 g flour
60 g butter
60 cl hot beef or veal stock
1 tablespoon pureed tomato
½ teaspoon prepared mustard
1 large onion, finely chopped
4 tablespoons soured cream

Prepare the meat at least 2 hours beforehand. First cut the fillet into slices, then into thin strips. If the meat is cut too thin, it hardens in cooking. Sprinkle with salt and pepper, and allow to stand. Meanwhile, brown the flour in half the butter, work to a paste gradually adding the stock, tomato puree, and mustard. Bring to the boil and strain. Allow to stand.

Twenty minutes before serving, fry the onion rapidly in the remaining butter with the meat, turning with a spatula, for 5 minutes and cooking only enough at a time to cover the bottom of the frying pan.

When the meat is ready, add soured cream to the sauce. Place, together with the meat and onion, in a firmly lidded pan. Stand it on a fireproof sheet on low heat for 10 minutes to keep warm. Bring to near boiling point just before serving.

If preferred, the sauce can be prepared omitting the mustard. Another variation adds thinly sliced mushrooms to the onions, which are then fried separately from the meat.