1. In a stand mixer fitted with the paddle attachment, combine the beef, pork, onion, breadcrumbs, egg, garlic, parsley, salt, allspice, nutmeg, and 2 tablespoons of the olive oil. Mix on medium speed until smooth and incorporated, about 5 minutes. 

2. Form the mixture into 18 meatballs, about 2 tablespoons each. Heat a large skillet over medium heat and add the remaining 2 tablespoons of olive oil. Working in batches if necessary, sear the meatballs on all sides until golden brown, about 2 minutes per side, 10 minutes total. Transfer to a paper towel-lined plate. 

3. In the same skillet, add the butter and heat until melted. Stir in the flour until a paste is formed. Continue to cook for 3 minutes, stirring constantly. Slowly pour in the beef stock and whisk until smooth. Bring to a boil and continue to cook, stirring often, until the sauce has thickened, about 10 minutes. Add the lemon juice and heavy cream and bring to a simmer. Season with salt and pepper. 

4. Add the meatballs to the gravy and continue to simmer until the meatballs register 155° F on an instant-read thermometer, 8 to 10 minutes. Serve over mashed potatoes and garnish with chopped parsley.