Lay flank steak toward the edge of a cutting board with the grain running vertically. Using a sharp knife held parallel to cutting board, cut along the side length of steak, working your way to split evenly in half cutting in the same direction as the grain. Leave about a 1/2-inch (1.25 cm) hinge of meat attached to open like a book. Cover with a double layer of plastic wrap; pound butterflied steak to an even thickness of about 1/2 -inch (1.25 cm).
Using the large holes on a box grater, grate 1/2 the onion into a 9 by 13-inch (23 by 33 cm) baking dish. Add orange zest and juice, lime zest and juice, 1/2 cup (about 15 g) chopped cilantro, garlic, chipotles, cumin, chili powder, Worcestershire and salt. Lightly whisk to combine all ingredients; lay flank steak into marinade. Flip flank to coat with marinade, cover with plastic wrap and marinate refrigerated 1 hour. Flip flank over in marinade, cover and refrigerate 1 additional hour. Dice second half of onion and soak in cold water 30 minutes (reserve for serving).
Preheat grill to high (500°F/260°C). Remove flank from marinade and place on paper towel-lined sheet pan; pat dry. Drizzle both sides of flank with oil, then place directly on the hottest spot of grill. Grill 1 minute, rotate 90 degrees and grill 1 additional minute. Flip flank steak; repeat process. Remove flank from grill and allow to rest 3 minutes. Slice with the grain to divide steak into 4 portions then thinly slice each portion against the grain. Season with finishing salt.
Drain onion and combine with remaining chopped cilantro. Cook tortillas. Serve sliced carne asada with onion-cilantro combination and corn tortillas.
