Preheat the oven to 350°F.
In a large bowl, combine the ground beef, ground pork, egg, Parmesan cheese, parsley, kosher salt, and black pepper. Mix together until just combined.
In a medium bowl, mix together the milk, breadcrumbs, garlic, and onion.
Add the milk mixture to the meat mixture, being careful not to overwork it.
Grease your hands to prevent sticking and form the mixture into meatballs of your desired size. Place them on a parchment-lined baking sheet.
Roast in the preheated oven until the outsides are browned and the internal temperature reaches 165°F on a meat thermometer, about 25 minutes.
Place the cooked meatballs into a slow cooker along with the grape jelly and chili sauce. Stir to coat the meatballs evenly.
Cook on low for 3 hours. Serve warm.