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1
Put oil in medium saucepan or skillet and turn heat to medium. When oil is hot, add onion, salt and pepper and cook, stirring occasionally, until it begins to brown at the edges, 5 to10 minutes. Add the chiles, tomatoes and 1/2 cup water and adjust the heat so the mixture simmers steadily but not violently. Cook about 15 minutes, stirring occasionally, or until the chiles are soft and the tomatoes are broken up. Meanwhile, start a charcoal or wood fire or preheat gas grill or broiler; the fire should be only moderately hot, and the rack about 6 inches from the heat source.
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2
When sauce is ready, taste, and add more salt and pepper if necessary. Cool for a few minutes, then remove the stems from the chipotles (if you remove some of the seeds the salsa will not be quite as fiery). Put the mixture in a blender and puree.
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3
Season pork with salt and pepper and grill for about 4 minutes per side, or until nearly done; brush heavily with the chipotle sauce on both sides, and cook just another minute until internal temperature on a thermometer reads 145 degrees Fahrenheit, followed by a 3-minute rest time.
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4
Serve pork garnished with cilantro and with the lime wedges, passing the remaining sauce of the side.
