For the sauce, cut the tomatoes into quarters (you do not need to peel them) and remove the hard white bits near the stem ends. Blend them to a light puree in a food processor. Heat the garlic and chile pepper in the oil in a wide skillet, stirring, for 30 seconds, until the aroma rises, then add the tomatoes, sugar, and some salt. Cook over mediumhigh heat, stirring occasionally, for 25 to 30 minutes, until the sauce is reduced and thick.
For the filling, fry the onion in the oil in a large skillet over medium heat, stirring often, until soft and golden. Add the tomatoes, sugar, oregano, and a little salt, and cook until the liquid has almost disappeared. Add the rice, stir well, then add the stock (or water and bouillon cubes). Stir in the saffron, and salt and pepper to taste, and bring to a boil, then simmer over low heat for about 15 minutes, until most of the liquid has been absorbed but the rice is still underdone.
Retaining the stalk, cut a circle around the stalk off the tops of the peppers, and keep these to use as caps. Remove the cores and seeds with a pointed spoon, and shave the inside of the lids to make more room for the filling. Using a spoon, fill the peppers with the rice without pressing it down too much, so that there is a little room for it to expand. Replace the caps and arrange the peppers snugly in a baking dish.
Pour the tomato sauce around the peppers, cover with a sheet of foil, and bake in a preheated 400 degrees F oven for 1 hour. Remove the foil and continue to bake for another 15 minutes, or until the peppers are soft and have browned on top; be careful not to let them fall apart. Serve hot.
Recipe courtesy of The Food of Spain by Claudia Roden, reprinted with permission from HarperCollins Publishers, copyright (c) 2011