Combine beef, pork, and onions in a large bowl using your hands or a spoon. Halfway through mixing, add garlic, Italian seasoning, salt, and pepper and mix well.

Combine bread crumbs, milk, and mayonnaise in a small bowl until you get a thick consistency, adding more milk if needed. Mix into meat mixture until thoroughly combined.

Form golf ball-sized meatballs and place on a plate or a pan.

Combine marinara sauce and water in a multi-functional pressure cooker (such as Instant Pot®). Place meatballs in sauce, making sure meatballs are covered at all times, adding sauce and stirring if needed. Close and lock the lid. Select high pressure according to manufacturer's instructions; set timer for 10 minutes. Allow 10 to 15 minutes for pressure to build.

Release pressure carefully using the quick-release method according to manufacturer's instructions, about 5 minutes. Unlock and remove the lid. An instant-read thermometer inserted into the center of a meatball should read at least 165 degrees F (74 degrees C).