In a sealable bag or airtight container, mix all the ingredients. Seal tightly and let marinate in the refrigerator for 8 hours or overnight. Remove the meat and onion from the marinade and pat dry. Discard the marinade.
Place a grill wok on the grill and preheat the grill, setting the burners to high. Oil the grate.
Place the sweet potatoes in the centre of two layers of aluminum foil. Lightly oil, season with salt and pepper, and seal the foil to make a parcel.
Place the foil pack on the grill, close the lid and cook for 20 minutes, turning the packet over halfway through cooking. Remove from the heat and mash the potatoes with a fork. Keep warm.
Meanwhile, grill the onion in the grill wok until it begins to colour. At the same time grill the meat for 3 minutes per side for rare. Season with salt and pepper. Let the meat rest on a plate for 5 minutes before serving. Heat the tortillas on the grill.
On a work surface, thinly slice the meat. Spread the sweet potato mash on the tortillas and top with the beef, onion and avocado. Serve with the sour cream, chipotle sauce and lime wedges, as desired.