Heat the oil in a large ovenproof skillet over medium-high heat until smoking. Sprinkle the chicken breasts on both sides with salt and pepper. Put them in the pan in a single layer, and brown them 2 minutes on each side. Put the skillet in the oven and roast until the chicken is just cooked through, about 15 minutes. Remove from the oven and cool.
Chop the cooled chicken and put it into a large bowl. Add the rest of the ingredients and toss. Season with salt and pepper.
Spread 4 bread slices with mayonnaise, if you like, and spread with the curried chicken. Top with another slice of bread and serve.