Heat 2 tablespoons of the olive oil over medium heat in a large nonstick skillet with the garlic until fragrant, about 1 minute. Stir in the fennel and onions and cook until brown and tender, about 8 to 10 minutes. Add the thyme, rosemary, lemon and orange peel, pepper flakes, olives, and tomatoes. Season with salt and pepper and cook until herbs and zest are fragrant, about 3 minutes more. Transfer vegetables to a serving platter and cover with foil to keep warm.
Wipe out the skillet.
Make 1-inch diagonal slashes into the fishes' skin to prevent the mackerel from curling while cooking. Add the remaining olive oil to the skillet over high heat and season the mackerel with salt and pepper. Place fish skin side down in the skillet and cook undisturbed until crisp brown and a spatula can easily be slipped under the fish to flip, about 2 to 3 minutes. Turn the fish over and cook another 2 minutes, until just firm. Set on top of the fennel mixture and serve.
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