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.