Preheat oven to 375℉. Line two large rimmed sheet pans with foil and grease lightly with cooking spray. Set aside.

Cut tomatoes into large chunks, anywhere from quarters to eighths, depending on the size of the tomato. Make sure all of the tomato pieces are similar sizes. Divide tomatoes evenly between the prepared baking sheets, making sure they are not too crowded. 

Peel onions and cut into large chunks. Scatter evenly amongst the tomatoes on the baking sheets. Smash and peel garlic cloves and add to the pans. Scatter oregano leaves evenly over the top of both pans.

Season both pans liberally with salt and drizzle evenly with olive oil. Roast for 45-60 minutes, or until the tomatoes and onions are charred in places and the juices from the tomatoes are caramelized.

Allow to cool slightly, then carefully transfer all of the roasted vegetables into a blender, along with the fresh basil. Carefully blend on high until smooth. Season liberally with fresh ground pepper and additional salt as needed.

Store in airtight containers in the refrigerator for up to a week or in the freezer for several months.