Put the grated onions into the centre of a piece of muslin and squeeze out as much of the juice as you can. Put the grated onion into a large bowl and add the ingredients up to and including the minced beef, then mix together with your hands. You really want to break the meat down so knead it for about 10 minutes until the mince is super-fine and the ingredients are well combined.
Work the chopped bone marrow into the meat so that it is all well combined and then work in the diced tomatoes. Divide the meat mixture loosely into 12 equal-sized balls. When you are ready to cook, prepare your barbecue for direct heat cooking. I use a plancha for burgers, but a good heavy-based frying pan will work fine too. Heat the pan or plancha until sizzling hot and add the oil. Place the chapli balls on the hot surface and smash them down using a burger press or suitable metal spatula. Placing a piece of greaseproof paper between the burger and the smasher will make this easier. Allow to cook for about 2 minutes before carefully flipping over. Place a piece of cheese on each patty and cover with a burger dome or just let the cheese melt naturally.
To finish, spread green chutney liberally (or to taste) over each bun half. Top the bottom buns with lettuce leaves and sliced tomato and onion. (Feel free to add any other toppings you like too.) Stack the cheese-topped patties on top of each other to make double burgers and then add the stacked burgers to each bun. Cover with the top bun and serve.