For the meatballs:
2 Tbsp vegetable or olive oil, divided
1/2 medium sweet yellow onion, finely diced
2 cloves garlic, minced
3/4 lb. ground pork
3/4 lb. ground beef
1/2 cup panko breadcrumbs
1/3 cup milk
1/4 cup parmesan cheese
1 large egg
1 tsp salt (or to taste)
1/2 tsp oregano
1/4 tsp allspice
1/4 tsp nutmeg
1/4 tsp fresh ground pepper (or to taste)
For the sauce:
1/4 cup butter
1/4 cup flour
2 cups beef broth
1 cube beef bouillon
1 Tbsp Worcestershire sauce
1 Tbsp fresh parsley, chopped fine (or substitute 1 tsp dried parsley)
1 tsp ground yellow mustard
1/2 cup sour cream
Heat 1 Tbsp olive oil in a large dutch oven and add the onions. Cook on medium heat for 4-5 minutes, to sweat and soften the onions. Add the garlic and cook one minute more, until aromatic. Transfer to a bowl and set aside to cool. Don't clean the pan.
In a large mixing bowl, combine breadcrumbs, milk, parmesan cheese, egg, salt, cooled onion mixture, and remaining meatball spices. Gently fold in the meat and mix to incorporate all the ingredients. Mix by folding and not squeezing, to avoid overworking the meat.
Use a 1 oz scoop or spoon to create 1 inch meatballs, rolling them gently into shape and setting them on a plate. Transfer to the fridge and chill for at least half an hour.
While the meatballs chill, make the base for the sauce by combining beef broth, bouillon, Worchestershire sauce, chopped parsley, and mustard powder. Whisk to dissolve the bouillon and set aside.
When the meatballs have chilled, add the remaining 1 Tbsp of olive oil to the pan used to sweat the onions and garlic, and heat on medium-high until a water droplet will sizzle when shaken into the pan. Add meatballs in batches and brown, being careful not to crowd the pan. Rotate often, to brown and cook all "sides" of the meatballs. (Adjust heat throughout so meatballs brown without burning.) When no pink spots remain, remove from the pan and set on a plate, continuing until all batches are cooked. (Meatballs might not yet be cooked all the way through. They will finish later in the sauce.)
Once the meatballs are browned and removed, begin the sauce in the same pan by adding butter and allowing it to melt. Stir in the flour and cook for about 2 minutes, stirring, until it begins to brown. (While the flour is browning, use a spatula to scrape any brown meat bits from the bottom of the pan. This is important flavor to make sure ends up in your dish!) Slowly add in the beef broth mixture, whisking while it is added, to prevent flour lumps from forming. Bring to a boil and then reduce to low heat to simmer. Place the sour cream in a bowl and "temper" it by adding a few tablespoons of the hot sauce at a time to it, stirring, to warm the sour cream slowly. (Adding cold sour cream to a hot sauce will curdle the sour cream. This is why we warm the sour cream first.) Add the warmed sour cream mixture back into the sauce.
Place the meatballs back into the sauce, gently, and spoon sauce to cover them. Also add any juices from the meatball plate! Allow the meatballs to simmer in the sauce for about 10-15 minutes until cooked through. Stir often, but gently, to make sure the sauce doesn't scald. If the sauce seems too thick, add a few tablespoons of milk to keep it thin enough to cook without sticking.
Serve over mashed potatoes or noodles, and garnish with more fresh parsley!