Heat a large skillet pan to medium-high heat. Season the beef well with salt and pepper, fry in the oil for a few minutes until browned all over, then remove from the pan.
Add the butter to the pan along with the sliced onions, mushrooms, and garlic. Cook for 5 to 10 minutes until the onions and mushrooms have softened.
Add the almond flour and the paprika to the pan and stir it in, thoroughly coating the onions and mushrooms.
Add the beef stock to the pan and stir everything together, then allow the mixture to cook and thicken for a few minutes.
Add the Dijon mustard, sour cream, cream cheese, and a dash or two of Worcestershire sauce to the pan, stir it in and allow everything to heat through.
Once the sauce has thickened, return the beef to the pan for a few minutes to warm through, then serve immediately with your chosen keto side dish.