Over a flame, in a broiler, on a grill or in a cast iron skillet on high heat, char skins of chiles until completely black on all sides. Put in a paper or plastic bag to steam for 15 minutes. Remove, then peel or rinse away skin; remove seeds and veins and slice into 1/2-inch-wide by 2-inch-long strips. In a large skillet over medium heat, heat lard or oil and cook onions, stirring occasionally, for 5 to 10 minutes, or until translucent and tender. Add chile strips and salt, and continue to cook, stirring occasionally, another 3 to 4 minutes.
Add potatoes and corn, and continue cooking for a few minutes, until potatoes are heated through. Check seasonings. If you are using crema, stir in at the last minute or so of cooking. Place in warm tortillas and add condiments as you wish.
For a nonvegetarian version, in the second step when cooking the onions, first cook about 8 ounces Mexican chorizo, squeezed from its skin, until its fat is rendered. Then add onions to the pan. Decrease the amount of lard or oil accordingly; you may need none if chorizo is very fatty.
Recipe courtesy of Mark Bittman