Hi Jamie
For a Chicken Tikka Main, the onions are usually raw, thinly sliced long, then spread onto the cast iron sizzler, heating on the burner.
The Tikka from the Tandoor or the Microwave is added on top and the heat is cranked up high, then the lemon dressing is squirted on to finish it off
and create the smoke, lovely aroma and drama as its carried through to the table.
(Everybody in the kitchen avoids this smoke, as its worse than a chilli cough, you literally have to dive for cover) 
For a mixed grill, big chunky onions and capsicum are deep fried quickly, then arranged on the
sizzler before being blasted and squirted.
Here's one I ate earlier
http://www.southtyneside.com/sizzler/sizzler.htmlcheers Chewy