Turn on your Instant Pot and select the 'Saute' function on high.
Add olive oil and onions and saute until onions are slightly softened. Add garlic and saute for an additional 2-3 minutes. NOTE: be sure to move the contents around in the Instant Pot regularly to ensure they are not sticking to the bottom of the pan.
Select the 'Cancel' function to reset the cooking function. Add tomato sauce or pasta sauce from the jar to the bottom of the Instant Pot and then add the Italian meatballs.
Add garlic pepper, Italian seasoning and pinch of red pepper flakes (optional).
Secure the lid on the Instant Pot. Select the 'Pressure Cook' function on high and cook for 10 minutes.
While the meatballs are cooking in the Instant Pot, toast the open face of your hoagie rolls with butter on the stovetop in a large skillet. I like to use butter sprinkled with a bit of garlic powder. Once they are taosted to your liking, put aside. You can also place the buttered hoagie buns in the oven to toast, as well, if prefered.
Once the timer beeps, allow to naturally release pressure for 10 minutes then quick release all remaining pressure.
Stir contents and serve meatballs on a tasted soft sub roll topped with Parmesan and mozzarella cheese.
Serve and enjoy!