In a food processor, combine one of the onion halves with the garlic, ginger, spice blend, salt, cinnamon, tomatoes, jalapeno, and the 1/2 cup water. Puree and set aside.
Thin-slice the remaining onion half. Film the bottom of a straight-sided 12-inch saute pan with the oil, and heat it over medium-high heat. Add the sliced onion and saute until it begins to color. Add the curry paste; reduce the heat to medium, and sauté for 10 minutes, stirring often with a wooden spatula, until the oil separates from the curry. Don't rush this step. Thoroughly sauteing the curry paste sets up the foundation of the dish.
Blend 2/3 cup of the yogurt into the curry sauce and simmer, stirring and scraping up the curry paste from the bottom of the pan, until the yogurt thickens and then nearly cooks away, 8 to 10 minutes.
Stir in the chicken, and add the remaining yogurt and the 1/3 cup water. Bring the mixture to a slow simmer. Cook, uncovered, for 8 minutes, or until the chicken is cooked through and tender.
Lift the chicken out of the pan with a slotted spoon, and place it in a serving bowl. Raise the heat until the sauce is boiling. Boil it down until once again it is so thick that the oil separates from the curry paste. Taste the sauce for seasoning, and pour it over the chicken. Sprinkle the curry with the fresh coriander.