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1
To make the chili sauce, in a dry heavy fry pan over medium heat, toast the chilies a few pieces at a time, pressing them flat against the hot surface with a metal spatula, until they are aromatic, about 10 seconds per side. (If the heat is right, you'll hear a slight crackle when you press them down, but you shouldn't see more than the slightest wisp of smoke; the inside surface of the chili should look noticeably lighter.) Transfer to a bowl and add hot tap water to cover. Place a small plate on the top to keep the chilies submerged and let stand for 20 minutes.
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2
Using tongs, transfer the chilies to a food processor or blender. Add the water, tomatoes and garlic and process to form a smooth puree. Set aside.
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3
To make the filling, in a 4-quart saucepan over medium-high heat, warm the oil. When the oil is hot, add the pork in a single layer (in batches, if necessary) and cook, turning, until richly browned all over, about 10 minutes. As the pork is browned, transfer it to a plate. When all of the pork is browned, return it to the saucepan.
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4
Push the chili-tomato puree through a medium-mesh sieve directly into the pan and cook, stirring frequently, until the saucy mixture is as thick as tomato paste, 6 to 8 minutes. Stir in the water and the 1 teaspoons salt, cover partially and bring to a simmer. Reduce the heat to medium-low and simmer, stirring frequently, for 20 minutes.
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5
Add the potatoes, cover partially and cook, stirring frequently, until the potatoes and meat are tender but not falling apart and the sauce has thickened to the consistency of canned tomato sauce, 20 to 30 minutes more. If the sauce is too thin, continue cooking, uncovered, until thickened. Taste and adjust the seasonings with salt.
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6
Scoop the hot pork-and-potato mixture into a serving bowl and sprinkle with the cilantro. Serve with the warm tortillas alongside.
