Halibut is the perfect partner for this robust sauce.
Drain tomatoes, reserving 1/2 cup liquid. Seed and coarsely chop tomatoes. Heat 1 1/2 tablespoons olive oil in a large skillet over medium-low heat. Add garlic, and cook until aromatic, 1 to 2 minutes. Add onion, and cook, until transparent, about 5 minutes. Raise heat to medium high; add tomatoes, reserved liquid, olives, capers, anchovies, and rosemary, and cook, stirring often, 2 to 3 minutes. Remove from heat, and set aside.
Heat remaining oil in a large nonstick skillet over medium-high heat. Season both sides of halibut steaks with salt and pepper. Cook steaks until golden brown, 4 to 5 minutes on each side.
Reheat sauce until it simmers. Stir in parsley. Serve halibut with a little sauce on each steak.