Heat the oil in a large skillet over medium high heat. When hot, sear the chicken on both sides (season the chicken with salt and pepper, and sprinkle it with half the curry powder). Once seared, remove from the pan and set aside.
Add the onions to the skillet (and optional raisins) and cook stirring occasionally, until translucent, about 5 minutes.
Turn the heat down to medium, sprinkle with the other half of the curry powder and continue to cook a minute or two.
Move the onion to one side of the skillet and add the chicken in one layer. Cook for about 15 minutes or until the chicken is cooked through. Remove chicken and set aside.
Add the sour cream to the skillet 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, turning once.