Preheat the oven to 350 degrees F.
Heat the oil in a medium heavy saucepan or skillet over medium-low heat. Saute the rice, stirring constantly, until golden brown and crackling, about 5 minutes. Add the onions and serranos and cook until the onions just soften. Then add the garlic and saute until the aroma is released, about 1 to 2 minutes. Pour in the hot stock or water and warm Red Tomato Salsa, mixing well to combine. Transfer to a 4-quart baking dish or casserole. Cover with foil and bake until the liquid is absorbed and the rice is tender, about 30 to 40 minutes. Stir, remove serrano chiles if desired, and serve hot.
Heat the vegetable oil in a medium skillet over medium heat. Cook the onions until soft, about 10 minutes. Transfer to a food processor fitted with the metal blade or a blender.
Add the tomatoes, tomato juice, garlic, and jalapeno and puree, in batches if you are using a blender, until smooth. Pour into a saucepan, and add the salt. Bring to a boil, reduce to a simmer, and cook, uncovered, 20 minutes. Adjust the seasonings. Set aside to cool for table salsa or use warm for rice or chilaquiles. Store in the refrigerator 2 to 3 days or in the freezer for weeks.
*To rinse rice, place the rice in a large bowl (not a colander) and rinse under cold running water for 5 minutes. (If you use a colander, the starch remains on the rice, leaving it sticky.)