Combine first 7 ingredients in a large zip-top plastic bag. Seal, and shake well. Remove tuna from bag; thread 4 tuna pieces, 3 red onion pieces, and 3 lemon wedges onto each of 8 (12-inch) skewers. Set tuna skewers aside.
Prepare grill. Place skewers on grill rack coated with cooking spray, and grill 4 minutes on each side or until tuna is medium-rare or desired degree of doneness. Serve with Tomato-Black Olive Relish.