Simmer the potatoes in salted water to cover until tender, about 15 minutes. Drain, cool under running water, peel and cut into 1/4-inch dice.
Heat the lard or oil in a heavy medium-size skillet over medium. Add the onion and potato, and fry, stirring regularly to ensure that nothing sticks to the pan, until the mixture is richly browned, 10 to 15 minutes.
While the potatoes are browning, seed the chiles and slice them into 1/8-inch strips. Stir them into the potato mixture and season with salt, usually about a 1/2 teaspoon. Remove the mixture from the heat and stir in the cheese. Scrape into a small dish and serve with tortillas for making tacos.