Fill a small saucepan with water and bring to a boil. Add the dried chiles and boil for 10 minutes, until the chiles are soft. Remove the pan from the stove and drain the water by pouring the contents through a colander. Rinse the chiles under running water. Use tongs to rinse away the loose seeds and pull away the stems. Discard seeds and stems.
In a medium sized skillet, heat the vegetable oil over medium heat. Add the chopped tomatoes and onions, and sauté for 1-2 minutes, until the onions are translucent. Add the corn tortilla masa mix (or flour) and stir to combine well. Once all of the dried masa is absorbed, carefully pour the chile puree into the pan. Stir to combine well.
Lower the heat and allow the sauce to simmer gently uncovered for 10 minutes, until the sauce reduces. Remove from heat.
Meanwhile, heat the broiler in your oven. Prepare the enchiladas by heating the corn tortillas on a griddle until they are pliable. Roll approximately ½ oz (14g) of cheese in each tortilla, and place in an oven-proof casserole dish. Once all of the enchiladas are formed, pour over the chile puree sauce. Top the enchiladas with the rest of the cheese.
Place the casserole under the broiler for 20-30 seconds until the cheese is just melted. Remove the casserole from the broiler, and top with the remaining chopped onions. Serve immediately.