Ingredients
2 medium onions, sliced 
2 tbsp oil 
2 garlic cloves, chopped 
2 tsp paprika 
200g mushrooms, sliced 
1 tbsp flour 
1 beef fillet weighing roughly 400g, or leftover roast beef, sliced 
1 tsp Dijon mustard 
1 tbsp tomato purée 
200ml beef stock 
150ml sour cream plus 4 tsp (for presentation)
½ tbsp lemon juice 
15g parsley, chopped 
Salt 
Pepper 
* 2 tsps porcini powder (optional)

Method
Put a wok or large frying pan on a medium heat and add 1 tbsp of the oil.  Add the onions and ½ tsp salt and cook for 10 minutes until caramelised.  Add the garlic and 1 tsp of the paprika and cook for another 2 minutes.  Put into a bowl and with kitchen towel just clean out your pan a little.   

Add 1 tbsp of oil to the pan and cook the mushrooms on a high heat until the juices come out of the mushrooms and gets reabsorbed - this will take 5-10 minutes depending on the mushrooms and how brave you are with the heat.  When you first start cooking you will need to constantly stir.  Put the mushrooms into the bowl with the onions.

Put the remaining 1 tsp of paprika to the flour with a tsp of salt and lightly dust your beef in the flour mix.  Add another tbsp of oil and flash fry the beef on high for 2-3 minutes.  Add the tomato purée and mustard and stir, add the beef stock and thoroughly stir then add back the vegetables, bring to the boil, reduce the heat and add the sour cream, lemon juice, porcini powder if using and the parsley. 

Check seasoning and serve either on rice or pasta with 1 tsp of sour cream and a dusting of paprika.