Marinate chicken breast with jerk seasoning, then grill thoroughly and set aside to cool. Peel, dice and cook the yam for about four minutes over medium heat, then drain and set aside to cool. Dice the pineapple, onion, pepper, scotch bonnet and scallions. In a bowl, combine olive oil, vinegar, onion, peppers, scotch bonnet and scallions, adding salt and pepper and stirring well. Add in the yam and pineapple, then stir gently. Place on a bed of lettuce, slice the marinated chicken and add overtop, then garnish with parsley.