Roast chiles over an open flame, or under a broiler, until blackened.
Place them in a plastic bag, sprinkle in a few drops of water then close the bag to allow the chiles to steam for a few minutes. Peel off the blackened skin, slice each chile lengthwise and remove the seeds.
Slice the chiles into long strips. Sauté with chopped onion, some garlic and diced tomatoes (if you want it very spicy, add a couple of roasted jalapeños to the mix).
When filling the tamales, sprinkle 2 to 3 tablespoons of shredded cheese on the spread masa and then spoon some of the chile mixture onto the cheese before wrapping.