Preheat oven to 375 degrees F. Line a baking sheet with parchment paper. Place the first 8 ingredients in a bowl and gently mix to combine. Moisten your hands with cold water. Roll the meat into 1 inch balls and place on baking sheet. Bake 20 minutes, or until cooked through.
While the meatballs cook, melt the butter in a pot set over medium-high heat. Add the onion and mushroom and cook 4 to 5 minutes. Mix in the flour and paprika and cook 1 minute more. While stirring, slowly mix in the stock. Bring to simmer; simmer 5 minutes. Whisk the sour cream into the sauce, season with salt and pepper, and then lower heat to medium-low.
When the meatballs are cooked, drain well and add to the sauce. Partially cover and let the meatballs soak up the flavour of the sauce for 10 minutes. Serve portions of the meatballs sprinkled with parsley.