1. Heat the butter in a large frying pan over medium heat. Add the onion and cook about 8 minutes, until golden. Add the mushrooms and cook about 5 minutes, until they begin to brown. Transfer to a plate.

2. Meanwhile, season the beef with salt and pepper. Mix the flour and paprika together in a large bowl, add the beef and toss well. Add the oil to the pan and increase the heat to high. In batches, add the beef and cook, stirring occasionally, for about 3 minutes, until the meat is seared. Transfer to a plate.

3. Return the beef, onions, and mushrooms to the pan. Stir over high heat for 1 minute. Reduce the heat to medium-low. Stir in the sour cream and mustard and heat, but do not boil.

4. Stir in the lemon juice and season with salt and pepper. Sprinkle with paprika and serve hot.

Variation

Veal Strogonoff

Substitute 16 whole peeled small shallots for the onions. Use 1½ lb (700g) veal filet instead of the beef. At the end of step 3, pour in 3 tbsp Cognac and immediately set it alight, shaking the pan until the flame is extinguished.