For the marinade: In a food processor, blend the garlic cloves, ginger, and cilantro to a paste consistency. Add the tamarind concentrate, ground turmeric, and chile powder to the paste, and stir well to combine. Rub the chicken pieces all over with the spice mixture.
For the xacutti masala: Heat a frying pan over a medium heat, add the sesame seeds, poppy seeds, chiles, cloves, pepper, fennel seeds, cinnamon stick, coriander seeds, cumin seeds, and unsalted peanuts, and dry fry for 30 to 45 seconds, or until the spices are fragrant. Using a spice grinder, transfer the mixture and grind the roasted spices to a fine powder.
For the sauce base: In a separate frying pan, heat 2 tablespoons oil over a medium heat. When the oil is hot, add half of the chopped onions, and fry for 4 to 5 minutes, or until the onions are golden brown. Add the grated coconut, and fry for another 3 to 4 minutes, stirring well. Remove the pan from the heat, and set aside.
In a separate pan, heat 3 tablespoons oil, add the remaining chopped onions and fry for 4 to 5 minutes, or until golden brown. Add the marinated chicken pieces, and any remaining marinade left in the bowl, and fry for 4 to 5 minutes, stirring continuously, or until the chicken is golden brown. Add the reserved coconut and onion mixture to the chicken mixture and stir well. Continue to cook the mixture for 3 to 4 minutes.
Add the xacutti masala mixture and stir until well combined. Bring the mixture to a simmer and simmer for 4 to 5 minutes, or until the oil separates from the rest of the mixture. Season the mixture with salt, to taste.
Add the tomato puree and the boiling water, and bring to a boil, stirring well. Reduce the heat to low, and simmer the mixture for 20 to 30 minutes, stirring frequently, until the chicken is tender, and the sauce has thickened.
To serve the dish, spoon the xacutti onto serving plates. Garnish with the chopped fresh cilantro, grated coconut, and lime wedges.