Start by mixing together all the spices, along with the lemon juice and crushed garlic cloves.
Cut the flank steak to thin strips, ensuring you cut against the grain.
Thinly slice the onions.
Place the flank steak in a bowl and pour the marinade on top, along with the onions. Cover tightly and marinate in the fridge for 2 hours.
Remove the shawarma from the fridge 1 hour before cooking.
Prepare the tahini sauce (see notes) and the sliced vegetables for serving with sandwiches.
Heat a cast iron skillet on medium high heat. Working in batches, and using tongs, add a few pieces of the shawarma on the hot pan, spreading them as much as you can. You can add the onions too.
Cook for 3-4 minutes on each side until a sear and caramelization is achieved on each side. Remove from the pan and rest for a few minutes.
Serve the beef shawarma in sandwiches, drizzled with the tahini sauce and along with your vegetables.
Notes
To make tahini sauce, which is the classic sauce for beef shawarma, just mix together 1 cup tahini, juice of 2-3 lemons, a minced garlic clove and salt to taste. Stir or whisk well with a fork (it will seize up but keep mixing) until its smooth. Drizzle it onto the beef shawarma
Make sure you marinate for 2 hours to get the best flavour
You can also serve beef shawarma over rice or a salad