2. Place the chopped tomatoes and onions into a roasting pan and toss with 2 or 3 tablespoons of olive oil.
3. Roast the tomatoes and onions until they cook down, any juices that have accumulated evaporate, and the mixture begins to caramelize or char - roasting for about 2 1/2 to 3 1/2 hours. Stir occasionally. The natural sugars in the tomatoes and onions will begin to develop.
7. Puree the mixture until it is well blended and nearly smooth. It will be thick. If using at another time, don't season the mixture until you are ready to use it for cooking. Don't add salt, pepper, herbs or oil. Wait to taste it and add what you need when you need it!