Step 1

Stir together beef, pork, eggs, parsley, Parmesan, panko, milk, garlic, and 1 1/2 teaspoons of the salt until combined. Shape mixture into 16 balls (about 3 tablespoons each).

Step 2

Select SAUTÉ setting on a programmable pressure multicooker (such as Instant Pot). Add oil to cooker. Select HIGH temperature setting, and allow to preheat, 2 to 3 minutes. Working in 2 batches, add meatballs to cooker. Cook, turning occasionally, until meatballs are browned, 6 to 8 minutes. Transfer cooked meatballs to a plate. Press CANCEL, and stir in 3 cups water and remaining 1/2 teaspoon salt, scraping to loosen browned bits on bottom of cooker.

Step 3

Add marinara sauce, spaghetti, and meatballs to cooker, stirring gently to combine. Cover cooker with lid, and lock in place. Turn steam release handle to SEALING position. Select MANUAL/PRESSURE COOK setting. Select HIGH pressure for 8 minutes. (It will take 14 to 15 minutes for cooker to come up to pressure before cooking begins.)

Step 4

When cooking is finished, carefully turn steam release handle to VENTING position, and let steam fully escape (float valve will drop). (This will take 6 to 8 minutes.) Remove lid from cooker.

Step 5

Stir spaghetti to unstick any noodles. Let stand 5 minutes. Serve with additional grated Parmesan.