Preheat the oven to 375 degrees F.
In small bowl combine the grapefruit segments, juice, and zest. Halve the fennel bulb lengthwise, thinly slice and add to the bowl. Add the olive oil, olives, parsley, 1/2 teaspoon salt, and red pepper flakes. Stir to combine.
Meanwhile, place the fish on a parchment-lined roasting dish. Sprinkle with remaining 1/2 teaspoon of salt and pepper and bake for 10 to 12 minutes depending on thickness.
Gently transfer the fish to a serving plate. Top with the grapefruit and fennel salsa and serve immediately.