Dice the chicken into chunks, around 2cm square. They don't have to be neat, but need to be of similar sizes.
Finely chop the garlic and then add a pinch of salt and pepper to it. With the back of a spoon or a knife, grind it all down even further.
Chop the onion into 8 equal pieces (cut in half, then each piece in half until you are left with 8). Put 7 pieces on one side for later. (These are for the kebab skewers.)With the one spare piece, 1 third of it off and dice that third really small. This is going in to the marinade. Put the other bit back with the other 7 pieces. In a food bag add the diced onion, garlic, olive oil, diced chicken and a pinch of salt and pepper. Mix it all in the bag and then leave to marinade for at least 30 minutes, but ideally between 1 and 24 hours.
Get your kebab skewers and load them up with the chicken, red pepper, courgette, onion pieces, mushroom and anything else you want on there. Alternate the chicken with veg so it isn't all bunched up together.
Cook on the BBQ on a low to medium direct heat for around 15 to 20 minutes - until the chicken is cooked through and the veg has a char colour to it. Serve with halloumi cheese, baked spuds and the rest of your BBQ assortment.