Cut the steak into slices 1/2 in thick, then cut each slice across the grain into strips 1/2 in wide. Dust with the flour to coat. (If you have velveted the meat this is already done.)
Divide the beef in 4 parts. In a wide pan over high heat, add 3 tablespoons of oil and quickly seal the slices of beef. Remove each batch and continue until all the meat is seared
When all the beef is sealed, Add it back to the pan, add the chopped onions and garlic and stir well. Let cook for 3 minutes.
Add the wine to deglaze the pan, stirring well to deglase the bottom of the pan and let cook over medium heat until the alcohol has evaporated.
Add the Paprika, the Dijon mustard, the ketchup, the Worcestershire Sauce, mix well and cook 2 minutes.
Add the stock, and the ketchup, mix and cook for 12 minutes. Season with salt.
In a sautée pan, over high heat add 1 tablespoon of oil and stir fry the mushrooms until golden.
In a bowl, add the flour or corn starch and add a little cream and whisk well so you don't get lumps. Add the remaining cream, whisking well. Add a laddle of hot Strogonoff sauce to the cream mixture and whisk.
Pour the cream and starch mixture back in the pot, add the Button mushrooms and mix well. Simmer over low heat for 10 minutes to cook the starch and mix all the flavours, stirring from time to time.
The sauce should thicken but be smooth, coating the meat. Remove from the heat and add the lemon juice and check the seasoning.
Garnish with minced fresh parsley, serve with Rice, Potatoes, or Noodles.