Butterfly pork tenderloin lengthwise. Flatten pork with the palm of your hand to achieve even thickness. Cover with oil and season with cumin, salt, and pepper.
Prepare grill for medium-high heat. Grill tomatillos, jalapenos, and onion for about 5 to 10 minutes, turning often, until soft. Remove vegetables from grill. Seed and stem jalapenos. Add tomatoes, jalapenos, and half the onions to a blender or food processor; blend until mixture becomes a thick paste. Transfer to a serving dish and mix in water and cilantro, adding salt to taste.
Place pork on the grill and cook for 3 to 4 minutes on each side, until internal temperature as measured with an instant-read thermometer is 160 degrees F. Slice pork and arrange on a warm serving tray.
Spoon some of the tomatillo mixture over the pork and decorate with the remaining grilled onions. Garnish with cilantro. Serve pork with remaining sauce.