Sprinkle the steak with the paprika and season generously. Heat half the butter and oil in a frying pan, add the steak and stir-fry for 4-5 minutes until sealed and lightly browned. Transfer to a plate and set aside.
Reheat a pan over a medium heat. Add the rest of the butter and oil and then add the shallot. Sauté for 2-3 minutes until softened but not coloured.
Increase the heat and add the mushrooms to the pan with the garlic. Season to taste and continue to sauté for another 2-3 minutes until tender. Sprinkle in the paprika and cook for another minute, stirring.
Add the white wine vinegar and allow to bubble right down, then add the wine with the stock, mustard and tomato puree, stirring to combine. Tip the sautéed beef back into the pan and simmer gently for about 5 minutes or until the sauce has reduced by half, stirring occasionally.
Stir the cream into the pan, bring to the boil, then reduce the heat and simmer gently for a couple of minutes and cook until sauce has thickened and slightly reduced. Divide the Dijon mash on warmed plates and spoon over the beef stroganoff. Add some long stemmed broccoli and serve at once.