Add the diced onions, ½ cup of vegetable broth, salt, and pepper into the Instant Pot. Secure the lid and place the steam release handle in the Sealing position.
Set the Instant Pot to normal saute mode for 5 minutes. When it is finished sauteing, carefully unseal the steam release handle. Once the steam has been released, remove the lid.
Stir the onions, then set the Instant Pot back to saute, this time with no lid. Add the garlic and tomato paste. Sauté the ingredients for 2 – 3 minutes, carefully stirring the bottom of the Instant Pot so that the ingredients don’t stick.
Pour in the red wine to deglaze the Instant Pot. Again, clear the bottom with a wooden spoon or soft spatula. Allow the wine to evaporate for 1 minute, then cancel the Sauté function.
Add the tomatoes, red chili flakes, basil stems, and vegetable broth. Stir everything well.
Place the lid on the Instant Pot, lock it into place, and then set the steam release handle in the Sealing position. Set the Pot to High Pressure for 25 minutes. Be sure to turn off the Warming setting.
Once the pressure cooking has finished, allow the steam to naturally release for 15 minutes. Then, unseal the steam release handle. When the steam releases, carefully remove the lid.
Stir the sauce and decide if you want it thicker (you will be pureeing it, which thickens it up). If you want it thicker, saute the sauce for 5-10 minutes with the lid off.
Let the sauce cool, and then transfer it to a bowl. Use an immersion (stick) blender to puree the sauce. You can also use a blender for this purpose, blending the sauce in batches if necessary.
Makes 7 cups of sauce.