Move the oven rack to the middle position and heat the oven to 475 degrees F.
Line a standard-sized rimmed cookie sheet with foil. Set wire rack inside a medium baking sheet and lightly spray rack with nonstick cooking spray; set aside.
MAKE MEATBALLS:
Melt 1 tablespoon of butter in a large skillet over medium-high heat. Add onions and cook until softened, about 8 minutes.
Transfer onions to a large bowl and add bread, ¼ cup broth, ¼ cup sour cream, yolks, allspice, nutmeg, salt, and pepper. Mix and mash with a fork until smooth and well blended.
Add ground beef and pork. Knead the mixture with your hands until the ingredients are well combined.
Roll the meat mixture into 1 ¼-inch meatballs (you should have about 60 meatballs) and place them on a prepared wire rack.
Bake until lightly browned, about 15 minutes. Transfer meatballs to a slow cooker.
MAKE SAUCE:
Melt the remaining butter in a large skillet over medium heat. Whisk flour into melted butter and cook until mixture starts to brown, about 3 minutes.
Slowly whisk in the remaining broth. Bring the mixture to a boil and pour it over the meatballs in the slow cooker.
SLOW COOK:
Cover and cook on low until sauce is slightly thickened and meatballs are tender, about 4-5 hours.
TO FINISH:
Ladle ½ cup of sauce from the slow cooker into a small bowl. Whisk in the remaining sour cream, soy sauce, and dill.
Stir the mixture back into the slow cooker.
Serve with extra dill as garnish.
Notes
This recipe makes about 60 meatballs. If you're serving this as an appetizer, it will serve about 12 people. If you serve it as a main dish over buttered noodles, it will serve 8-10 people.
Look for the rye bread in the grocery store's bakery section, not on the sandwich bread aisle.