Put the ginger, garlic, 1/4 tsp of salt, and lemon juice into a food processor. Add about 2 Tbs of water and blend until smooth, pushing down with a rubber spatula, if necessary. Place the chicken pieces in a non-metallic bowl. Pour the ginger mixture on top of the chicken and rub in. Cover the bowl with plastic wrap and refrigerate for 30 minutes or for up to 24 hours.
Without cleaning out the food processor, add tomato, cilantro, chilies, tomato paste, the remaining 3/4 tsp of salt and 2 Tbs of water. Blend until smooth.
Pour the oil into a large, non-stick, lidded pan and set over high heat. When very hot, put in the chicken pieces, together with the remaining ginger marinade. Fry, turning the chicken periodically, until the chicken pieces are light brown on all sides, about 10 minutes. Add the tomato mixture from the food processor. Continue to cook, stirring, until the sauce thickens and clings to the chicken and the oil separates, another 10 minutes. Add the yogurt. Stir and cook until the yogurt is well blended and disappears, leaving a thick sauce edged with oil, about 4-5 minutes. Cover, reduce heat to as low as possible and cook for 10-15 minutes, or until chicken is tender. If the sauce is drying out too much, stir in 1-2 Tbs. of water.