Preheat oven to 400 degrees F. Line a ¼ size baking sheet pan or other small sheet pan with parchment paper. Set aside.

In a large bowl make the meatball mixture. Combine ground beef, ground pork, onion, seasoned bread crumbs, parsley, egg, Worcestershire sauce, garlic powder, kosher salt, and pepper. Try not to overmix the mixture or the meatballs will be tough.

Using about a tablespoon of the meatball mixture, form into balls. Place on the prepared sheet pan. The mixture should make about 30 meatballs.

Bake the meatballs in the oven for 16 to 18 minutes or until the internal temperature is about 160 degrees F on a instant read digital thermometer.

While the meatballs bake, prepare the sauce. In a large sauté pan, melt the salted butter. Add the onions and mushrooms and sauté, stirring occasionally, 4 to 5 minutes. The onions and mushrooms should be just soft, but still holding their shape.

Add the tomato paste and Dijon mustard. Stir and cook for about 1 minute.

Sprinkle the onion mushroom mixture with the all purpose flour. Stir to combine and cook for 1 minute.

Slowly add the reduced sodium beef broth and stir to combine. Bring to a low boil and cook the sauce until slightly thickened, about 3 to 4 minutes.

Remove the pan from the heat and stir in the sour cream and parsley. Season to taste with kosher salt and pepper. Add the baked meatballs to the sauce.

Serve over egg noodles, rice, or mashed potatoes.