Let the tomatoes cool in their roasting pans unless there's a lot of burned juice on the bottom, in which case dump them into a large bowl or two. In the food processor, coarsely puree the celery, carrots, jalapeno pepper, and the roasted onions. If sticking occurs, add a little of the reserved tomato juice. Meanwhile separate the garlic cloves, squeezing the roasted garlic out of the peel, and add it to the vegetables in the pot. If you're using fresh oregano, tie the bunch up in a white string so you can remove it easily later.
Oregano stems are nasty little sticks.
Add the tomatoes, sugar, salt, and oregano to the pot and cook slowly, uncovered, stirring from time to time to prevent sticking and to break up the tomatoes. Cooking time will depend on how much juice was left in your tomatoes and how thick you want your puree to become. I usually cook the sauce about 2 hours. After an hour or so of cooking, if the mixture seems thin and astringent, add 1 or 2 tablespoons of canned tomato paste. Stir in well and continue to cook at least another half hour.