Clean and completely dry off the chicken pieces, they should not have any excess moisture. The reason we mix thigh and breast pieces is because you need the slight fat content that is in the thigh meat to give moist results. So make sure not to trim off all the fat from the chicken when cleaning. If your chicken pieces do not have any bits of fat, add 1 tsp. of butter to compensate for it.
Put the chopped onion in a processor and grind to a rough consistency. Remove and squeeze out all the excess water. Then put the onion back in the processor, add the chillies, coriander, mint, spices, salt, ginger, garlic and lemon juice. Whizz it to combine, then add the chicken cubes.
Grind the mixture until combined and smooth, but take care not to over process as that will end up making the chicken too soft to handle.
Turn the mixture out into a bowl, and add the egg gradually while mixing. You may not need the full egg, once the mixture is slightly sticky, stop adding egg. If the mixture ends up being too soft, add the breadcrumbs and combine well. You can also add some cheese for added flavour if you want.
Cover the bowl and refrigerate the mixture for 2 hours. Refrigerating helps to firm up the mixture so that it is easier to handle.
When ready to cook, scoop handfuls of the mixture and form sausages on skewers. You can also shape these on the stick end of a wooded spoon for an authentic seekh shape, just be sure to grease it lightly so that the kabab slides off easily after shaping. Heat a drizzle of oil in a non stick pan or grill pan and arrange the kababs on it. Cook for about 10 minutes or as needed, turning them as needed to brown the seekh kababs evenly.