Trim the touch silver skin from the pork tenderloin with a sharp knife. Place tenderloin in a shallow baking dish. Mix the oil with the garlic, cumin, and salt. Rub the tenderloin with the mixture. Cover and refrigerate for 30-60 minutes, or overnight. Let tenderloin come to room temperature before grilling.

Preheat grill to medium-high heat. Oil the grates well. Place tenderloin on the grill in the hottest part. Sear it on all sides. Move to the cooler part of the grill and let it continue cooking for about 10 more minutes. Test for doneness by using an instant-read thermometer. When it reaches 140°F in the center, remove from grill and place on a platter. Cover and let stand for 10 minutes.

While tenderloin is grilling, place the whole poblano chiles and thick onion slices on the oiled grill. Turn the peppers every few minutes allowing the skin to blister a bit. They may pop and sputter a bit. Just turn them consistently so they don't burn on any of the sides. Remove to a platter and cover. Keep warm. Let the onions cook for about 5 minutes on each side. They will blacken in spots, but not soften too much. Transfer to a platter and keep warm.

Clean the grill briefly and oil it again. To warm the tortillas, place them briefly on the grill, turning after 30-60 seconds, being careful not to let them fall between the grates.

To serve: Mix the sour cream with the milk and chipotle puree together in a small bowl. Cut the pork tenderloin into thin slices on the diagonal, against the grain. Cut the peppers and onions in thin strips. Place a few slices each of the tenderloin and veggies into the tortillas and drizzle with some of the chipotle crema. Eat while warm.