In a bowl combine meatball ingredients (beef, pork, onion, garlic, breadcrumbs, egg yolk, milk, parsley salt and pepper). Mix well to combine
Form meat mixture into about 20 round meatballs. Place meatballs onto a baking sheet or plate lined with parchment paper. Refrigerate for 1 to 2 hours.
Set Instant Pot to Sauté. Add half meatballs and brown on all sides. Remove and set aside. Repeat browning process with remaining meatballs. Set aside.
Add stock, butter, soy sauce, and mustard to Instant Pot and mix. Add meatballs and cook on high pressure for 5 minutes. Let natural release for 10 minutes.
While meatballs are cooking combine cream and flour in small bowl.
When pressure is released, set Instant Pot to Sauté. In a small pan or skiller add lemon juice and cream and flour mixture to Instant Pot. Stir to combine and bring to simmer stirring frequently. Continue cooking until sauce thickens. Taste for seasoning. Add meatballs and combine.
Serve with noodles or mashed potatoes.