1. Mix breadcrumbs and 1/3 cup stock in a small bowl. Set aside. Melt 1 tablespoon butter in a medium skillet over medium heat. Add onion and sauté until browned, about 10 minutes. Transfer onion to a large bowl.
2. Wipe out pan and return to medium heat. Add bacon and cook until crisp. Using a slotted spoon, transfer bacon to bowl with onion. (Reserve bacon fat.) Add next 8 ingredients to bowl with onion mixture, mixing with your hands to blend. Fold in breadcrumb mixture. Using a 1 tablespoon measure, roll meat mixture into balls; transfer to a rimmed baking sheet.
3. Melt 1 tablespoon butter with reserved bacon fat in a large heavy pot over medium-low heat. Working in 3 batches and adding 1 tablespoon butter between batches, brown meatballs on all sides, about 6-8 minutes per batch. Transfer meatballs to a plate. Drain all but 2 tablespoons drippings from pot. Whisk in flour until smooth paste forms. Stir in 2 cups stock; bring to a simmer, whisking often. Return meatballs to pot. Cover; simmer until meatballs are cooked, 5-6 minutes. Remove from heat, whisk in sour cream, and stir to coat meatballs.
