Prepare a grill or grill pan for medium-high heat; lightly oil the grill grates.
Toss together the olives, parsley, vinegar, tomatoes and 2 tablespoons of the oil in a large bowl. Season lightly with salt and generously with pepper and toss again; set aside until juicy.
Meanwhile, brush the fish on both sides with the remaining 1 tablespoon oil and season with salt and pepper. Grill the fish, skin side up, until it has strong grill marks and releases easily from the grill, about 5 minutes. Rotate it about 45 degrees to create a crosshatch pattern and continue to grill for another 3 minutes. Turn the fish over and cook until just opaque throughout and an instant-read thermometer inserted in the side registers about 135 degrees F, 5 minutes more. Transfer to a cutting board.
Using the natural seams of the fish as a guide, cut into 4 pieces. Put a slice of bread on 4 serving plates. Top with a piece of fish and spoon the tomato mixture and its juices over the top.
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