Sear the sliced beef in 1 tbsp butter until well cooked. The beef does not have to be very browned, just cooked through. Remove and set aside.

Deglaze the pan with enough water to loosen everything up. Add mushrooms and 1 tbsp butter. Cook on medium until mushrooms are chestnut brown. Do not stir a lot so they will brown well. Remove and set aside with beef.

Add the onions, beef stock, thyme, nutmeg, about 1/2 tsp salt, and several grinds of black pepper. Simmer for 15 minutes so the onions break down. Cook pasta at this point if serving with.

Once the liquid has reduced down by over half, add the beef and mushrooms back in and simmer another 5 minutes.

Add the sour cream and stir well. At this point, never let the sauce go above a light simmer or the dairy will curdle. If the sauce is too thick, add 1 tbsp water at a time. If thin, add 1 tbsp corn starch slurry (corn starch + cold water). Continue until it coats a spoon.

Serve over pasta or along side french fries. Optionally garnish with chopped fresh parsley.