1. Prepare a grill for high heat cooking. Brush the grill grate with oil. Grill the tomatoes, jalapeno, and onion, turning frequently, until the vegetables are charred all over. When cool enough to handle, stem and seed the jalapeno (leave the seeds in for a hotter salsa), and trim the root end from the onion. Chop the vegetables with a knife or pulse them briefly in a food processor. Stir in the cilantro, garlic, and salt and pepper to taste.
2. Meanwhile, brush the pork with oil and sprinkle with chile powder, salt, and pepper. Grill the pork, turning occasionally, until an instant-read thermometer inserted into the thickest part of the pork registers 145 degrees F, 20 to 25 minutes. Transfer the pork to a cutting board and let rest 10 minutes. Slice the pork and serve with the salsa.
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