Toss the chicken pieces with salt and pepper.
Roughly chop the onions and cilantro.
Split the mangoes to remove the stones then score the flesh with a sharp pointed knife to create cubes. Scoop out the flesh with a spoon.
Heat the cooking oil in a wide frying pan. Place the chicken pieces in a single layer, skin side down, in the hot oil. Do NOT move them around for several minutes. Just leave them alone for a while and give them a chance to brown. You have to cook the chicken over high heat to brown the outside.
After letting the chicken fry WITHOUT TOUCHING THEM for about five minutes, check to see how they are browning. If the skins are nicely browned, flip to brown the opposite side.
Scatter the chopped onions, cubed mangoes and chopped cilantro over the browned chicken pieces.
Pour the yogurt over the chicken, mangoes and vegetables. Sprinkle about a teaspoonful of grated lemon zest all over. There is no need to stir at this point. Cover the cooking pan, lower the heat and simmer for an hour to an hour and a half, depending on how large or small your chicken pieces are. Leave them alone, I tell you. Do not be tempted to peek and stir. You want the juices retained in that covered cooking pan.
When the chicken is almost done, stir the stew, cover the pan once more and turn up the heat to medium. Continue cooking for another 5 to 10 minutes or until the sauce is reduced to almost nothing.