In a medium bowl, combine the chicken pieces with the turmeric, red chili powder, lemon juice, ginger-garlic paste, and ¾ teaspoon of salt. Mix well to coat and set aside to marinate while you prepare the spices.
Place a dry skillet over medium-low heat. Add the dried red chilies, coriander seeds, fennel seeds, cumin seeds, peppercorns, cardamom pods, cinnamon stick, cloves, star anise, and optional stone flower. Toast, stirring frequently, until fragrant and a shade darker, about 5-7 minutes. Transfer to a plate to cool completely.
In the same skillet, add the shredded coconut, cashews, and one sprig of curry leaves. Toast over medium-low heat, stirring, until the coconut is light golden brown, about 4-5 minutes. Remove from heat.
Once the whole spices are cool, grind them into a fine powder using a spice grinder. Add the toasted coconut mixture and ¼ cup (60ml) of water to the grinder or a small blender. Blend until you have a smooth, thick paste.
Heat the oil in a large pot or Dutch oven over medium heat. Add the chopped onion and cook, stirring occasionally, until softened and golden, about 8-10 minutes.
Add the marinated chicken to the pot. Increase the heat to medium-high and cook until the chicken is lightly browned on all sides, about 5 minutes.
Stir in the chopped tomatoes and cook until they soften and break down, about 5 minutes. Add the prepared Chettinad masala paste and the remaining sprig of curry leaves. Stir constantly and cook for 2 minutes.
Pour in the hot water and stir to combine, scraping any browned bits from the bottom of the pot. Bring to a simmer, then reduce the heat to low, cover, and cook for 15-20 minutes, or until the chicken is tender and cooked through.
Uncover and check that the chicken has reached an internal temperature of 165°F (74°C). If the sauce is too thin, simmer uncovered for a few minutes to thicken. Taste and adjust with more salt if needed before serving.