Wash and prepare the tomatoes (I always cut out that little part at the top where the stem was attached).
Once the water is at a full boil, carefully place the tomatoes in the water for about 40-60 seconds, or until the skins start to peel away.
Remove them from the water and allow to cool enough to handle them.
Slip the skins off and place the tomatoes bowl. (discard the skins)
Add the oil to your IP insert and press the "Sauté" button.
Stir in the onions and sauté them until they become translucent.
Turn the IP off and add all the other ingredients EXCEPT the fresh basil.
Push the "Manual" button on the IP and adjust time to 15 minutes.
Once the IP finishes cooking, do a quick release with the vent valve.
Remove the lid and use a potato masher to mash the tomatoes.
Stir in the fresh basil while the sauce is still hot.
Transfer to a holding container until you are ready to use the sauce. Make sure to store in the refrigerator.
FOR THE STOVE TOP:
If you want to make this on the stove, blanch the tomatoes as instructed. Sauté the onions in a large stock pot, then add everything else into the pot and cook until the tomatoes are easily mashed with a potato masher. (about 30-40 minutes give or take. As soon as you turn off the heat, stir in the basil and then mash everything.
Add salt to taste at the table.
Please note that the nutrition data is a ballpark figure. Exact data is not possible.