Heat up your grill at medium high heat.
While the grill heats up, slice the onion in half horizontally, leaving the skin on and root base and top in tact. These will be the base and top of your makeshift rotisserie.
Insert one half of the onion onto three skewers evenly spaced in a triangle shape so that the skewers can stand upright with the onion at the base - the pointy ends facing up.
Start to layer the chicken pieced onto the skewers, you may need to fold pieces over onto themselves to be able to thread them onto the skewers. The idea is that the chicken should fit snugly inside the rough circumference of the onion. You don't too many loose bits hanging out as these will burn too quickly.
Once all the chicken is on the skewers, top with the second half of the onion to cover the pointy skewer ends. The chicken should be evenly spaced and snug, but not too tight, on the skewers.
Lie your makeshift shawarma rotisserie on the heated grill on its side. Turn down the heat to medium low (or place it on the cooler side of your grill).
Cover and cook for about 35-40 minutes, turning every 5 to 8 minutes to ensure the chicken gets golden brown and crisp all over. The best way to know that the chicken is cooked, is to check the internal temperature is at least 165F (74C).
Depending on how hot your grill is, you may find that the outside cooks too fast, in that case, you can shave off the cooked bits as you go, and then keep them aside while you continue cooking the rest - in true shawarma style!