Place the chicken, 3 tablespoons of the vegetable oil, the curry powder, salt, and cayenne in a bowl and stir until the chicken is thoroughly coated with the oil and spices. Refrigerate, covered, for at least 1 hour and for up to 8 hours.
Heat a large skillet over high heat and, when hot, add the remaining tablespoon of vegetable oil. Add the chicken and cook, stirring occasionally, until golden brown around the edges and cooked through, 5 to 6 minutes. Using a slotted spoon, transfer the chicken to a clean large bowl and set aside to cool.
Lower the heat to medium-high and add the onion and garlic to the skillet. Cook, stirring occasionally, until the onion is translucent, 3 to 4 minutes. Add the onion and garlic to the chicken and set aside to cool before proceeding.
Add the apple, lime juice, mayonnaise, sour cream, grapes, and raisins to the bowl and stir to mix well. Serve immediately, or refrigerate, covered, until ready to serve. Garnish with the cashews just before serving. This salad is best served within 24 hours.
Makes 6 to 8 servings