Heat ½ of the olive oil in a large, high-walled skillet and add the finely diced onions and garlic. Soften for 5 minutes. Set aside and let cool.
In a large bowl, combine the breadcrumbs, Parmesan cheese, whisked egg, milk, cooled onions/garlic, salt, oregano, allspice, nutmeg, and pepper. Gently incorporate the meat until evenly combined, but without overworking it (otherwise the meatballs will be tough instead of tender).
Roll into 1 ½-inch meatballs and place on a plate. (A small cookie scoop makes it easier to roll balls of uniform size.) Transfer to the fridge and let them chill for 15 minutes, or up to overnight. Cover if chilling overnight.
While the meatballs chill, combine the beef broth, chicken bouillon, Worcestershire sauce, Dijon, and parsley in a large measuring cup with a spout. Set aside.
Heat remaining olive oil in a large skillet over medium-high heat.
Brown the meatballs in batches, leave plenty of room around each so that you can easily rotate them with a spoon to brown on all sides. Brown for about 1 minute per side. Remove and set aside. Add additional oil and/or decrease heat slightly as needed throughout cooking.
Melt the butter in the same skillet and use a silicone spatula to “clean” the bottom of the skillet. Stir in the flour and cook for 2 minutes, stirring continuously, until it begins to brown.
Add the beef broth mixture in small splashes, stirring continuously. Bring to a boil, then reduce to a simmer.
Place the sour cream in a medium bowl. Spoon some of the sauce into the bowl (about ¼ cup), and stir until well-combined. Stir it into the sauce over low heat until incorporated.
Add the meatballs back to the skillet along with any juices from the plate. Spoon the sauce on top. Allow them to heat through over low heat for 10-15 minutes (or until cooked through), partially covered. Garnish with fresh parsley and serve over mashed potatoes or egg noodles.