Firstly, I highly recommend making this in advance to develop the flavours.
In a small pot combine 3 tsp garam masala, 2 tsp chilli powder, 1.5 tsp cumin, paprika & turmeric, 1 tsp ground coriander and 1/2 tsp salt. Place to one side.
Butterfly your breasts right through to make 4 even sized breasts. Coat in 2 tbsp veg oil and HALF your spice mix (save other half for later).
Heat up a large pan over high heat and fry your chicken breasts until nicely charred on each side and hot through the centre (some char = authentic flavour). Place to one side to rest, scraping out any little bits of char/chicken as well. Important to rest for the chicken to retain moisture and completely cool. Once cool, dice into tiny cubes.
In a medium sized mixing bowl combine 1/2cup/120g yogurt and mayo, 1 tbsp tomato puree and finely diced coriander/cilantro, 1 finely diced shallot, red pepper, garlic, the juice of half a lemon and 1/2 tsp sugar.
Stir in diced chicken then season to taste. Cover in cling film and rest in the fridge until needed, then serve as a sandwich, in a pita/wrap or on a baked potato!