Instant Pot Instructions:

1. Pour 1 ½ cups water into the Instant Pot. Place a trivet in the bottom of the pot. 
2. Add the beef, pork, bread crumbs, eggs, onion, salt, pepper and half of the tomato sauce into a mixing bowl. Use clean hands to combine the mixture. 
3. Divide the meatball mixture in eight equal portions. Divide the cheese into eight equals portions. Roll into balls (the balls will be about the size of a tennis ball). Make an indentation in each meatball. Press a portion of cheese into each indentation. Press the meatballs to close securely.
4. Place meatballs on top of the trivet. 
5. In a small bowl whisk together the remaining tomato sauce, vinegar, brown sugar, mustard and Worcestershire sauce. Divide the mixture in half. Brush half of it on top of the meatballs. Save the other half for later. 
6. Cover the Instant Pot and secure the lid. Make sure valve is set to sealing. Set the manual/pressure cook button to 5 minutes. When time is up let the pot sit for 5 minutes and then move the valve to venting. Remove the lid.
7. Brush the rest of the sauce on top of the meatballs. Use tongs to place meatballs on plates. Eat and enjoy!

Slow Cooker Instructions:

1. Add the beef, pork, bread crumbs, eggs, onion, salt, pepper and half of the tomato sauce into a mixing bowl. Use clean hands to combine the mixture. 
2. Divide the meatball mixture in eight equal portions. Divide the cheese into eight equals portions. Roll into balls (the balls will be about the size of a tennis ball). Make an indentation in each meatball. Press a portion of cheese into each indentation. Press the meatballs to close securely.
3. Place meatballs into Instant Pot. 
4. In a small bowl whisk together the remaining tomato sauce, vinegar, brown sugar, mustard and Worcestershire sauce. Pour the mixture over the meatballs.
5. Cover and cook on high for 3 hours or on low for 4-6 hours (or until internal temp is 145 degrees). Remove the lid.
6. Use tongs to place meatballs on plates. Eat with the sauce and enjoy!