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Drain the chiles, remove and discard the stems and seeds, and place in a small saucepan along with the serranos, garlic, and half of the oil. Bring just to a simmer over medium heat; reduce the heat to low and let cook for about 20 minutes, until the garlic is soft.
Transfer the chile mixture to a food processor, add the coriander and cumin, and pulse until pureed. Drizzle in the remaining oil and pulse once more to combine; set aside to cool. Use within 2 hours, or cover and refrigerate for up to 1 day. (Makes 1 cup.)
To make the rib roast: Remove the roast from the refrigerator and let it sit at room temperature for 45 minutes.
Meanwhile, prepare a grill using natural charcoal. When the coals are very hot, push them to one side of the grill to create a hot zone for direct grilling and a cooler zone for indirect grilling.
Generously sprinkle all sides of the roast with salt and pepper. Using the hot zone of the grill, sear the meat on the bone side for 1 or 2 minutes, until well browned, then turn the meat with tongs and sear it on the narrow sides of the roast for 1 to 2 minutes on each side. Finally, sear the meaty side for about 3 minutes.
Move the roast to the cooler zone of the grill and place it bone-side down. Brush the meat with a generous layer of chile paste, reserving up to 1/4 cup to use prior to serving. Close the lid and grill for 1 1/2 to 1 3/4 hours, until an instant-read thermometer inserted into the thickest part of the meat, but not touching the bone, registers to 125 degrees F for rare or about 130 degrees F for medium-rare.
Transfer the roast to a cutting board and brush with more chile paste. Let the meat rest for 30 minutes before carving. Serve with the remaining chile paste on the side.