In a small bowl, whisk together lemon juice, garlic, basil, and mustard. In a slow, steady stream, whisk in olive oil. Pour into a large resealable plastic bag. Add shrimp, tomatoes, red onion, and zucchini. Chill for 2 to 4 hours.
Place wooden skewers in enough water to cover; soak for at least 30 minutes.
Spray grill rack with nonflammable cooking spray. Preheat grill to medium-high heat (350° to 400°).
Drain shrimp mixture, discarding marinade. Thread onion, tomatoes, zucchini, and shrimp onto skewers.
Arrange skewers on grill, and grill for 3 to 4 minutes on each side or until shrimp are done. Garnish with lemon slices, if desired.
