Directions:
Season the beef to taste and roll into 1-inch balls.  Heat the vegetable oil in a skillet over medium heat and cook, stirring occasionally, until the meatballs are browned on all sides and cooked through, about 15 minutes.  Remove the meatballs to a warmed serving dish and keep warm.

Wipe out the skillet, melt the butter over medium-low heat, add the onion, and cook until softened, about 5 minutes.  Reduce the heat to low and stir in the flour with a wooden spoon until it is well blended.  Let cook gently for about 3 minutes.

Pour in the stock and bring to a boil so that it thickens, reduce the heat, and add the mushrooms and Worcestershire sauce.  Let simmer 5 to 7 minutes longer or until the mushrooms are just cooked through.

Remove the skillet from the heat and stir in the sour cream.  Add the meatballs and heat gently so that the sauce can warm through.  Season to taste with more salt and pepper, sprinkle with the parsley, and serve.

For the Crème Fraîche:

Whisk 1 cup of whipping cream until very slightly thickened.  Fold in 1 tbsp buttermilk.  Mix the two together in a large bowl, and pour into a lidded glass jar.  Shake well and leave on the kitchen counter until thickened, usually a day or two, then put the jar in the fridge where it will keep 7 to 10 days.  It gets tangier as it ages.