Set oven to bake at 200 degrees. Prepare baking sheets by spraying with a non-stick cooking spray.

Cut tomatoes in half, and lay skin side down on baking sheets. Sprinkle with olive oil and lightly salt.

Put in oven, for 2-4 hours (depends on size of tomato). Check periodically. When tomatoes have lost most of their water content, but before they brown, they are done. 

Let cool, then store in airtight container in fridge (up to 4 days for optimal freshness), or freeze on the tray. Once frozen, store in freezer bags for up to a year.