Heat a grill to medium-high and spray with cooking spray. (If you don't have a grill, you can use the broiler.)
Sprinkle the fish with salt and pepper.
In a medium bowl, combine the olives, tomatoes, olive oil, parsley and lemon zest.
Grill the fish until it flakes easily when tested with a fork, about 4 minutes per side. Divide the fish between the plates, top with the olive relish and sprinkle with the feta.