How to Make It
Preheat the oven to 350°. Lightly oil a 9-by-13-inch glass or ceramic baking dish. In a medium skillet, warm the 1/2 cup of vegetable oil over moderate heat. In another medium skillet, warm 1/2 cup of the Salsa Roja over moderate heat. Using tongs, dip the tortillas in the oil, coating both sides, until softened, about 5 seconds. Coat each tortilla with the Salsa Roja and transfer to a plate, stacking the tortillas on top of each other.
In a medium bowl, mix the queso blanco with the Monterey Jack. Set a tortilla on a work surface and spoon 1/4 cup of the cheese and about 1 tablespoon of minced onion in the center. Loosely roll up the tortilla like a cigar and set it in the prepared baking dish, seam side down. Repeat with the remaining tortillas, cheese and onion. Pour the remaining 3 cups of Salsa Roja over the rolled enchiladas and sprinkle the remaining 1 cup of cheese on top.
Bake the enchiladas for 25 minutes, or until heated through and the sauce is bubbling. Scatter the cabbage and tomatoes over the enchiladas and serve hot.