Core and peel the tomatoes. Remove the tomato seeds and pulp. Put seeds and pulp through a food mill. Chop the outsides of the tomatoes
In a large pot over low heat, boil the milled liquid and tomato paste for 1 hour, or until the liquid is reduced by 1/3.
In a blender or food processor, puree cilantro, jalapeno, garlic and 1/2 cup of the tomato liquid from the large pot. Slowly stir this mixture into the pot.
Bring the contents of the pot to a boil while mixing in the green peppers, onions, chopped tomatoes, salt, vinegar and brown sugar. In a small bowl, dissolve the cornstarch in water. Add the cornstarch mixture to the pot. Stirring continually, boil the mixture until all the vegetable are soft -- about 30 minutes. Allow the mixture to cool, and place it in sterile containers.