Melt the butter in a Dutch oven or similar-size pot. Stir in the flour with a whisk and cook over medium-high heat, frequently stirring with the whisk, until the roux turns a gentle brown colour, about 5 minutes. Add the stock and cook, stirring continuously, until the sauce thickens, 6 to 8 minutes.
Turn off the heat and whisk in the crème fraîche and Worcestershire sauce. Season with salt and pepper and cover the pot.
Coat the skillet you used to cook the onions with enough vegetable oil, so there is a good oil film on the bottom. Then, working in batches, fry the meatballs and crowd them in the pan. (Unless you have a particularly huge skillet, you should plan to fry these in three or four batches.)
As the meatballs cook, turn them with a spatula or firmly shake the pan to brown evenly on all sides. Add them to the sauce when the first batch of meatballs are browned on all sides and fully cooked. Wipe the skillet clean with a paper towel between batches and cook the rest of the meatballs, adding more oil to the pan, if necessary, and transferring the finished meatballs to the sauce.
Gently rewarm the meatballs in the sauce. Spoon the meatballs and some of the sauce over a plate of warm noodles. Sprinkle chopped parsley over the top and serve a spoonful of lingonberry jam alongside.