In a blender or food processor, whirl garlic, green onions, chilies, cumin, oregano, and vinegar until coarsely pured.
Rinse flank steak and pat dry. Rub garlic mixture over meat.
Peel red onion; cut crosswise into 1/2-inch slices. Rub lightly with olive oil.
Cut rolls almost in half horizontally, leaving attached on one side. Pile into a basket.
Lay steak and onion slices on a barbecue grill over a solid bed of very hot coals or high heat on a gas grill (you can hold your hand at grill level only 1 to 2 seconds); close lid on gas grill. Cook meat, turning to brown evenly, until as done as desired in center of thickest part (cut to test), 12 to 15 minutes for rare, about 20 minutes for medium-rare. Grill onion slices, turning with a wide spatula, until lightly browned on each side, about 13 minutes.
Transfer meat and onion to a carving board.
Serve hot, warm, or cool.
Cut beef across the grain into thin, slanting slices. Tuck slices of meat and pieces of red onion into rolls. Spoon chipotle crema into rolls, and add tomatillo slices, if desired.
