Place the mince into a large bowl with all the the onion, garlic, garam masala, coriander and chilli flakes and a good pinch of salt. Mix well with your hands. Pat the mix into small burgers.
To cook, heat grill to its highest setting and lay the burgers in a single layer on a baking tray (you may need to do this in batches, depending on how big your tray is).
Grill on the highest shelf for 5-6 minutes on each side until browned and cooked through. Pile burgers onto a platter and serve with all the accompaniments, so everyone can construct their own sandwich.