1. Soak the bread in the milk. Peel and quarter the onion.
2. Place the onions in the bowl fitted with the ultrablade knife and mix at speed 12 for 10 seconds. Add the veal, pork, oregano, soaked bread, egg yolk, 1/2 tsp. salt and 1/4 tsp. pepper. Pulse for 5 seconds. Scrape down the sides of the bowl and pulse for another 5 seconds or until mixture is combined. Form mixture into 24 balls and chill for 30 minutes.
3. Wash the bowl and attach the whisk. Add flour, butter, white wine, broth, 1/2 tsp. salt and 1/4 tsp. pepper. Mix at speed 7 for 30 seconds. Launch the sauce program at speed 4 at 190°F for 5 minutes. Add the sour cream 2 minutes before the end of cooking. Mix at speed 4 for 20 seconds.
4. Cook the meatballs for 7-10 minutes in a large quantity of chicken broth until no longer pink in the center. Serve with the sauce.