Do not add any red chilli powder to this dish as it will change the color of the curry. For extra spice, use red chili flakes, green chilies or pepper powder.
Heat oil in a pan, followed by the whole spices (cumin, cardamom, cinnamon). Let them splutter then add the chopped onions and fry until slightly golden, then add the ginger and garlic pastes. Fry for a minute, then add the chicken and increase the heat to medium high. Fry for about 5 minutes until chicken turns white, then slowly add the sour cream/yoghurt and the coconut milk. Stir well and lower the heat to low, cover the pan and let these simmer for 10 mins, giving a stir every now and then.
Check the gravy, if you feel you prefer it with a bit extra gravy, then add as much water as you need, and then mix the 1 tsp. of flour/corn flour with a bit of water and add it to the dish. This will help thicken the gravy slightly. Add the slit green chilli. Continue to simmer for another 10-15 minutes.
Once the chicken is nice and tender, turn off the heat, then add the chopped coriander, salt and lemon juice. If you notice, the salt is added right at the end, this helps keep the chicken very moist, flavourful and tender. If you add the salt too early, the chicken pieces will be dry and rubbery.
Give the chicken about 5 minutes to let the flavours infuse before serving, you can garnish with some almonds if you like. Serve with rice and naan! :)