The chicken tikka sandwich is a problem before it is a pleasure: the meat that defines it is cooked to be dry. Tikka is marinated in yoghurt and spice and then taken over fierce heat until the surface chars and the flesh sets firm with no sauce clinging to it. That is exactly what makes it good off a skewer and exactly what makes it difficult between two slices of bread, where a filling with no internal moisture reads as spiced gravel and the bread stays a separate, untouched layer. The whole craft of this sandwich is solving the dryness without drowning it, and the build is judged on whether it manages that or not.
The fix is a bound moisture layer, not a wet one. The tikka pieces are cut down so no single bite is all charred edge, and they are carried by a thick, clinging dressing rather than a thin sauce: a mayonnaise loosened with the tikka spices, or a tight yoghurt mixture stiff enough to coat the meat and bridge it to the crumb without running to the corners. The bread is soft and plain so it yields to a filling that brings no give of its own, and it is buttered to the edges first, both to seal the crumb against the spiced dressing and to add the fat that dry meat needs. Salad leaf or cucumber goes in for water-crisp and a cooling break, kept dry so it does not undo the butter's barrier. Pressed and cut, the test is a bite that tastes of char and spice but holds together as a sandwich rather than spilling dry cubes out of the side.
The variations move the moisture problem around rather than away. Chicken tikka and mango chutney leans the whole answer on a sweet fruit counter; chicken tikka and raita leans it on a cooling yoghurt one; the chicken tikka wrap solves containment with a flexible bread instead of a rigid one; the mass chiller version standardises the dressing for a shelf life. Each deserves its own article rather than being crowded in here.