Preheat the oven: Set to 400°F (200°C).
Line a baking dish with parchment or lightly oil it.
Prep the base: In the baking dish, toss tomatoes, olives, and garlic with olive oil, lemon juice, lemon zest, red pepper flakes, a big pinch of salt, and black pepper. Spread into an even layer.
Season the fish: Pat halibut dry. Season both sides with salt and pepper.
Place fillets on top of the tomato-olive mixture.
Bake: Cook for 10–14 minutes, depending on thickness. Halibut is done when it flakes easily and is opaque in the center. Aim for an internal temperature of 130–135°F (54–57°C) for juicy results.
Rest and finish: Let the fish rest 3 minutes.
Spoon the pan juices over the top. Scatter fresh herbs and add extra lemon if you like.
Serve: Pair with crusty bread, couscous, roasted potatoes, or a simple green salad.