Preheat the oven to 200°C. Rub the chicken breasts with the olive oil, then season well. Heat a heatproof frying pan over a moderate heat until hot, then sear the chicken breasts until golden brown all over. Transfer to the oven to finish cooking for 15-20 minutes. Meanwhile, preheat the grill to hot. Toast the bread for 2 minutes on each side until lightly coloured. Combine the tomato and mayonnaise in a small mixing bowl with some seasoning. Cover and set to one side until needed. Check the meat is cooked through with no pink showing. Remove the chicken when cooked, allow to rest for 5 minutes, then slice crossways into strips. Top the toasted bread with the tomato mixture, then with slices of the chicken. Flash under the grill for 1 minute, then transfer to serving plates. Sprinkle a pinch of oregano on each crostini and garnish with a sprig of parsley. Serve immediately.
