Preheat broiler with rack about 8 inches from heating element.
Make diagonal slashes ¼ inch deep at 2-inch intervals across flesh side of fish.
Stir together garlic, oregano, lemon zest and juice, 1 teaspoon salt, ¼ teaspoon pepper, and 2 tablespoons oil; spread on both sides of fish, rubbing into slashes.
Cut fennel stalks from bulb; reserve ¼ cup of picked fronds.
Place stalks lengthwise on a rimmed baking sheet. Top with fish, slashed side up.
Cut fennel bulb in half lengthwise; remove and discard core.
Thinly slice bulb lengthwise and toss with tomatoes, olives, remaining tablespoon oil, ½ teaspoon salt, and ¼ teaspoon pepper; spread evenly around fish.
Broil until fish is just cooked and opaque throughout and vegetables are charred in places, 8 to 10 minutes. (If they begin to blacken, tent with foil.)
Divide fish and vegetables among 4 plates; garnish with fennel fronds.
