Cut the beef into 1cm thick slices and then into 1cm wide strips across the grain.
Cut potatoes into 3mm thick sticks and soak in cold water.
Heat sunflower oil to 190°C in a large pan or deep-fat fryer.
Melt butter in a frying pan and cook onion with paprika until soft.
Add mushrooms, fry for three minutes, then set aside on a warm plate.
Drain the potatoes and dry them thoroughly using a tea towel.
Fry potatoes in hot oil for three minutes until golden and keep warm in a low oven.
Heat 1.5 tablespoons of oil in the frying pan until very hot.
Fry the steak in two batches for one minute per batch, seasoning as you go.
Combine the onion mixture with soured cream in the pan and simmer until thickened.
Return the steak to the pan and heat gently for one minute.
Stir in lemon juice and parsley, then serve with the hot potatoes.