You know that these will be a hit at your party, because they have the word "party" in the name. Millions of people in the 70s knew it wasn't a good time unless a chafing dish full of these meatballs was making an appearance. How many other dishes do you know that guarantee a good time?
For the meatballs: Preheat the oven to 425 degrees F. Line a sheet pan with aluminum foil, then place a wire rack over it.
Grate the onion into a large bowl using a box grater. Add the ground beef and pork, tomato soup, panko, mustard powder, pepper, salt and minced garlic and knead together with your hands until thoroughly mixed. Use a tablespoon to help you make small meatballs, placing them close together but not touching on the prepared sheet pan. Bake until well browned, 15 to 20 minutes.
For the sauce: Melt the butter a large saute pan over medium heat. Add the onion and cook until soft and translucent, 3 to 4 minutes. Stir in the flour until no raw bits are visible, then continue cooking for another minute. Whisk in the beef stock 1/2 cup at a time, allowing it to thicken between each addition, then whisk in the grape jelly until smooth. Cook until the sauce begins to bubble, then add the graham cracker crumbs, ketchup and vinegar. Reduce the heat to medium-low and continue cooking, stirring occasionally, until it thickens slightly, 3 to 5 minutes.
Add the meatballs to the saute pan and gently toss to coat. Cook for another 3 minutes, then transfer to a chafing dish. Sprinkle with chopped parsley and serve warm.