Put the oil in a large skillet over medium high heat. When hot, add the onions (and optional raisins), sprinkle with some salt and pepper, and cook, stirring occasionally, until translucent, about 5 minutes. Turn the heat down to medium, sprinkle with half of the curry powder, and continue to cook a minute or two.
Meanwhile, season the chicken with salt and pepper to taste and sprinkle it with the remaining curry powder. Move the onion to one side of the skillet and add the chicken in one layer. Cook for about 2 minutes on each side. Transfer to a plate.
Add the sour cream and stir constantly over medium-low heat until the mixture is nice and thick. Return the chicken to the skillet and cook for a couple more minutes, or until cooked through, turning once.
Garnish with cilantro or parsley and serve with rice.