COMBINE tomato, onion, bell pepper, lemon juice and pesto in a serving bowl; cover and refrigerate until serving time.
ADD the shrimp to a large bowl. Add the olive oil, lemon juice, mustard and garlic. Toss well. Cover and refrigerate for 1 hour.
REMOVE the salsa from the refrigerator and bring to room temperature. Preheat grill to medium-high. Remove the shrimp from the marinade; discard the marinade.
THREAD 4 shrimp per skewer, piercing them through both ends (this will help to keep them flat). Grill for 3 to 4 minutes on each side. (Or, alternately, line a boiler tray with foil. Preheat broiler. Broil the shrimp about 6 inches from the heat source, turning once, for about 4 to 5 minutes or until they just turn pink and are no longer translucent.) Serve the salsa with the shrimp.
NOTE: If using wooden skewers, soak in water for 30 minutes before threading.