Step 1Combine the garlic and ginger with about 1 tablespoon of water in a small food processor and process to a paste. Set aside.
Step 2Heat 1 teaspoon of oil in a Dutch oven over medium-high heat. Add the onions, tomatoes, garlic and ginger paste, cinnamon stick, cardamom pods, cloves, cumin seeds, coriander, turmeric and salt. Cook 10 minutes, stirring occasionally.
Step 3Add the chicken and stir to mix with the spices. Cook 15 minutes uncovered, stirring occasionally, then cover and cook 5 minutes. Add the coconut milk and water. Cover and cook 10 minutes.
Step 4Meanwhile, heat the remaining 1 1/2 teaspoons of oil in a skillet over high heat until very hot.Add the mustard seeds (be careful, they'll pop out of the skillet), chiles and curry leaves. The oil should be hot enough so the curry leaves crackle and turn black right away; the chiles should also turn black. Cook no more than 3 minutes. Pour this mixture into the chicken. Simmer 5 minutes longer.