Press the Sauté button on the Instant Pot.
Once heated, add the sausage (or beef) and onion, breaking the meat up with a wooden spoon. Cook for 4-5 minutes, until the meat is no longer pink, then add the garlic and sauté for 1 minute, seasoning with the Italian seasoning, salt, and red pepper.
1 pound ground Italian sausage, ½ small red onion, 3 cloves garlic, 2 teaspoons Italian seasoning, 1 teaspoon kosher salt, ¼ teaspoon crushed red pepper flakes
Press the Cancel button, then add in the sauce, stirring well.
24 ounces marinara sauce
Break the spaghetti in half and spread them in an even layer over the sauce mixture. Do not stir.
8 ounces spaghetti noodles
Add the remaining water to the pasta sauce jar, close, shake gently, and pour the saucy water over the pasta. Again, do not stir.
1 ½ cups water
Close the lid and secure the pressure valve. Cook on Manual High Pressure for 8 minutes. When the cooking time is up, perform a Quick Pressure release.
Carefully remove the lid and stir the cooked spaghetti into the sauce.
Serve immediately with Parmesan and your favorite pasta sides.