In a measuring cup combine mustard, lemon juice, olive oil, Worcestershire, marjoram and black pepper.
Place the thin chicken breasts into a bowl, pour the marinade over prepared chicken and toss to coat. Leave the chicken on the counter for at least 15 minutes.
Preheat a grill to medium/medium-high and grill for 6 to 8 minutes a side. Adjust the heat accordingly.
Remove to a cutting board, tent with foil and allow the chicken to rest for 5-8 minutes before serving.