In a large bowl, combine ground beef, ground pork, Panko, egg yolks, green onions, parsley, allspice and nutmeg; season with 1 teaspoon salt and 1/2 teaspoon pepper.
Using a wooden spoon or clean hands, stir until well combined. Roll the mixture into 1-to-1-1/4-inch meatballs, forming about 24 meatballs.
Heat olive oil in a large stockpot or Dutch oven over medium high heat. Add meatballs, in batches, and cook until all sides are browned, about 2-3 minutes. Transfer to a paper towel-lined plate; set aside.
In a large stockpot or Dutch oven of boiling salted water, cook pasta according to package instructions; drain well.
Melt butter in the Dutch oven. Whisk in flour until lightly browned, about 1 minute. Gradually whisk in beef stock and cook, whisking constantly, until slightly thickened, about 3-5 minutes.
Stir in meatballs and cook, stirring occasionally, until heated through and thickened, about 5 minutes.
Stir in pasta and sour cream until heated through, about 2-3 minutes; season with salt and pepper, to taste.
Serve immediately.
Notes
Prep the meatballs ahead of time. The meatballs can be assembled up to 1 day in advance and placed in the fridge until ready to use. The uncooked meatballs can also be stored in the freezer for up to 1 month (once flash freezing), thawing overnight in the fridge.
Store-bought meatballs also work in a pinch. Use your favorite store-bought frozen meatballs in a pinch, cooking to package instructions.
Use tube pasta. Tube pasta such as elbow macaroni, penne, rigatoni and ziti are great options. The hollow center allows to effectively capture and hold onto the cream sauce gravy, while also creating more surface area for the sauce to cling to compared to other pasta shapes.
Use good-quality stock. The better quality your stock, the better your dish. Many brands love to pack in the salt so always opt for unsalted or a lower-sodium variety.
Make it lighter. For a lighter option, substitute plain Greek yogurt for the sour cream.
Serve with crusty bread. Serve with all the homemade crusty bread for dipping, sopping and dunking!