This open-faced sandwich is fantastic! The recipe is adapted from one in a book given to me several years ago by my mother-in-law called "Season's Greetings", by Marlene Sorosky. Marinating time is not included in prep time.
To make the marinade, mix the oil, garlic, mustard, salt, lemon juice, vinegar, and cayenne in a medium glass or plastic bowl. Stir in the sliced onion and shrimp. Cover and marinate in the refrigerator up to 3 hours.
Using your hands, remove as much bread as possible from the inside of the half-loaf of French bread, leaving a 1-inch rim. Place the bread pieces in a food processor (fitted with metal blade) and process into crumbs; measure 1 cup. Saute' the 1 cup crumbs in 2 tablespoons of the butter until golden; set aside.
Preheat oven to 400°F With a fork, mix remaining 4 tablespoons butter with 3 tablespoons parsley and the garlic in a small bowl until combined. Spread on the inside of the hollowed loaf. Remove the shrimp and onions from the marinade (reserving marinade) and place in the loaf. Sprinkle with the olives and pimento, then drizzle with 3 tablespoons marinade (discard remaining marinade). Sprinkle with the sauteed bread crumbs and remaining 2 tablespoons parsley.
Place the bread on a sheet of foil in the center of the oven. Bake for 18-20 minutes, or until the shrimp are pink and the bread is crusty (watch closely--mine did not take this long).
Using a sharp, serrated knife (or a large mezzaluna, as I used), cut into 6 slices to serve. Serve warm or at room temperature.