Cut the pork fillet into 3-4cm (1½"-1¾") cubes and season with salt.
Heat a 28cm or 11" frying pan (not nonstick if possible) over medium-high heat, add the butter, and when it begins to foam, throw in the mushrooms and pork. Cook for 3-4 minutes, stirring occasionally.
When the pork and mushroom mix has had its cooking time, turn the heat up to high and add the brandy. *See Note*, then reduce the brandy until there is a tablespoon or so left.
Pour in the stock and add in the sour cream, mustard, and paprika, reduce the heat to medium and stir to combine.
Mash around one-third (1 tablespoon) of the butter with the flour and set it aside.Add this mix (beurre manié) and stir for a couple of minutes until the sauce begins to thicken, then remove from the heat for a minute or two; this will thicken the sauce a little more.
Finally, sprinkle with diced cornichons which will cut through the richness beautifully.
Video
Notes
Be careful when adding the brandy, if you are using an open flame or your pan is very hot it could catch fire. It is not a problem, it is called flambe and it is fancy, but be aware!