In a medium bowl, combine beef, pork, eggs, Italian seasoning, garlic powder, celery seed, onion powder, smoked paprika, and oats.
Form into 20 meatballs, then press 1 mozzarella ball into the middle of each of the meatballs.
Select Sauté on the Instant Pot and heat 2 tbsp avocado oil. Place 10 of meatballs around the edge of the Instant Pot and sear all sides of the meatballs for 3–4 minutes.
Remove the meatballs and place into a large heatproof bowl (that will fit inside your Instant Pot – see step 6) then add the remaining 1 tbsp avocado oil and sear the remaining meatballs for 3–4 minutes too.
Once all browned, place all the meatballs in the bowl and add the chopped tomatoes.
Press cancel on the Instant Pot and add 2 cups water inside and add the trivet. Place the glass bowl with meatballs and tomato sauce on top of the trivet and secure the lid.
Select Pressure Cook, and set the time to 20 minutes.
When the cooking program has finished, let the pressure release naturally for 10 minutes, then quick-release any additional pressure until float valve drops and then unlock lid.
Skewer a meatball, a basil leaf, and a tomato slice on a toothpick. Repeat 20 times.
Drizzle skewered meatballs with olive oil and balsamic vinegar. Serve immediately.