Make the Panade: In a large bowl, combine the panko, milk, and heavy cream. Let sit for 10 minutes so the breadcrumbs fully soften. This keeps the meatballs tender.
Cook the Onion: Melt the butter in a skillet over medium-low heat. Add the grated onion and cook for 7–8 minutes until soft and lightly golden. Stir in the garlic during the last 30 seconds. Remove from heat and let cool slightly.
Mix the Meatballs: Add the egg yolks, cooked onion mixture, parsley, salt, white pepper, black pepper, allspice, and nutmeg to the panade. Mix well. Add the ground beef and veal and gently mix until just combined. Avoid overmixing to keep the meatballs tender.
Shape: Roll the mixture into 1½-inch meatballs.
Brown the Meatballs: Heat the neutral oil in a large skillet over medium heat. Brown the meatballs on all sides until deeply golden, about 7–9 minutes total. Work in batches if needed. Transfer to a plate; they do not need to be fully cooked yet.
Make the Roux: In the same skillet, melt the butter over medium heat until lightly golden and nutty. Whisk in the flour and cook for 1–2 minutes, stirring constantly.
Add the Broth: Slowly whisk in the beef broth and chicken broth until smooth. Scrape up any browned bits from the bottom of the pan for extra flavor. Simmer for 3–5 minutes until slightly thickened.
Finish the Gravy: Stir in the heavy cream, soy sauce, Worcestershire sauce, and Dijon mustard. Reduce the heat to low and stir in the sour cream until smooth. Season with salt and white pepper to taste. Do not boil after adding the sour cream.
Finish Cooking: Return the meatballs to the skillet and spoon the gravy over them. Simmer gently for 8–10 minutes until fully cooked and tender.
Serve warm over mashed potatoes or buttered egg noodles with lingonberry preserves and fresh parsley, if desired.