Add another small splash of olive oil to your pan or instant pot. Then add the tomato paste, salt and pepper to the onions and cook for an additional 2 minutes.
Stir in the crushed tomatoes and the basil.
If your sauce is not already in your slow cooker (or instant pot), transfer it to your slow cooking appliance.
Add in all your meatballs to the sauce and gently push them down into it without mashing them together too much. Don't worry if the sauce doesn't totally cover all of them.
Set the instant pot to the slow cooker setting on HIGH (the "more" setting), close the lid, set the seal to "venting," and cook for 3 hours. After 3 hours, open the pot, gently stir the sauce being careful to keep the meatballs intact, and readjust your slow cooker heat to LOW (the "normal" setting). Slow cook on low for 2 more hours.
Start boiling your water and cooking your pasta about half an hour before the meatballs and sauce are ready (at the 4 hour 30 minute mark).
Serve the sauce and meatballs over the spaghetti. Garnish with grated Parmesan cheese and parsley, if desired.
Notes
If you like a little spice, you can add a teaspoon or two of red pepper flakes to your sauce before starting your 5 hours of slow cooking.