Place chicken in a bag and pour 200 ml of balsamic vinaigrette in bag with chicken. Marinate for at least 2 hours, but best if marinated overnight. Take chicken out of bag and discard marinade. Grill chicken breasts, about 5 to 6 minutes on each side until cooked through. Place yellow bell pepper slices in grill pan and grill about 2 minutes on each side. Once cooled, dice chicken up in small bites. Whisk together remaining 100 ml balsamic vinaigrette dressing and mustard in a small bowl. Prepare the salad leaves in a bowl. Toss together the leaves, chicken, balsamic mixture, spinach, and remaining ingredients. Serve immediately.