Direction :

1. Place the tomatoes, onions, kosher salt, and butter into your stovetop pressure cooker. Close the lid. Turn the heat to high. When the two red lines of the valve are visible, reduce heat to low—the valve should drop to reveal only 1 line—and cook for 5 minutes. Turn heat off. Let valve release naturally, which should take between 15-20 minutes.
2. Remove lid. Purée, with the basil if using, until smooth—I use an immersion blender for this; if you don’t have one, use a traditional blender or a food processor. Sauce will be thin. Return pan to burner and simmer over low or medium heat until sauce is desired consistency. Note: keep in mind, when you use sauce in something like eggplant involtini or eggplant parmesan, it will reduce further as it cooks. More and more I keep the sauce on the thin side and reduce it as needed. Also, this recipe could easily be turned into soup by simmering it down to the desired consistency and adding a splash of cream. 
3. Store in quart containers in the fridge for up to a week; freeze for 3 to 6 months.