I have posted my favorite recipe. It is some work and very much dominated by Indo/Pak flavors so try it only if you like that food. You asked for a killer sandwich and this is all our family favorite. If I place these on a party platter they are gone in less than 15 minutes.
Classic Bun Kabab
For the Patty
Ground Beef and chickpeas (authentically we use chana dal but you can use chickpeas in a can) cooked together with little water, minced ginger, minced garlic, salt, red chili powder and garam masala. Once the water dries (ensure mixture is not too wet) take it out in a bowl and add lots of freshly chopped cilantro, freshly chopped mint, 1 finely diced jalapeno and freshly roasted and powdered cumin and some bread crumbs. Combine and form into large patties.
Dredge in semolina, dip in egg and pan fry until nice and crisp.
You can make this ahead of time and freeze
Now the assembly
Kaiser Rolls (Large ones)
Smear one side with cilantro mint chutney (to make use a few bunches of cilantro, 1 large bunch of mint, several cloves of garlic, 1 jalapeno and juice of 2 limes along with some water and blend into a thick paste)
Dash of ketchup on the other side. Place the patty, add tomato and onion slices. Pan fry them in a little oil and viola ready to cut and serve.
It is work but very different and very good and yes there is garlic galore in it along with raw onions so you need a mint afterwards