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Wash and wrap the potatoes in aluminum foil. Bake the potatoes until tender but still firm, 1 to 1 1/2 hours. Cool the potatoes until they can be easily handled.
Meanwhile, roast the chiles over an open flame until completely blackened all over, 10 to 15 minutes. Place the chiles in a paper bag or in a bowl covered with plastic wrap to steam until the skin loosens, about 20 minutes. Wipe away the charred skin with a dry towel but do not rinse. Remove the stems and seeds and dice into 1/2-inch pieces.
Peel and dice the cooled potatoes into 1-inch pieces. Heat a large nonstick saute pan over medium-high heat and add the potatoes. Add enough clarified butter or lard to come about one-quarter of the way up the sides of the potatoes. Leave the potatoes undisturbed until they become golden and develop a crisp texture, 7 to 10 minutes, then flip and brown on the other side, 7 to 10 minutes more. Remove the potatoes and drain the fat from the skillet.
Immediately add the chorizo and onion to the pan. Cook until the fat is rendered and the chorizo begins to crisp slightly, 3 to 5 minutes. Then add the garlic and cook for another 3 to 5 minutes.