1 lemon, juiced
Salt and pepper, to taste
2 tablespoons Dijon mustard
1 clove garlic, finely chopped
1/4 cup olive oil
4 split bone-in chicken breasts or skinless boneless breast halves
1 French baguette
1/4 cup chopped fresh parsley
1/2 cup mayonnaise
In a bowl, whisk together the lemon juice, salt, pepper, mustard and garlic. Whisk in the oil a little at a time until the dressing emulsifies. Place the chicken breasts in a large bowl or baking dish. Pour the marinade over them and turn them in the dressing. Cover with plastic wrap and refrigerate for 2 hours.
Heat a charcoal or gas grill. When the grill is medium-hot, place the rack over the coals. Set the bone-in breasts skin side down on the rack. Cover and grill them for 10 minutes; do not move the chicken until it releases easily from the rack when you nudge it with tongs. Turn the chicken, cover the grill, and continue to cook for 8 minutes or until the meat is cooked through.
If using boneless chicken, place the breasts on the rack skinned side down. Cover and grill them for 5 minutes. Turn the chicken, recover the grill, and cook for 5 minutes more or until the meat is cooked through.
Transfer the chicken to a cutting board and let it cool.
Slice the meat. Cut a French baguette lengthwise, then crosswise into 4 pieces. Toast the bread lightly. Stir the chopped parsley into the mayonnaise and add a squeeze of lemon juice. Spread the bread with the parsley mayonnaise mixture, fill it with shredded lettuce and sliced chicken, and serve at once.