2. Heat the olive oil in a large skillet over medium heat. Add the onion and cook, stirring occasionally, until it is soft and translucent, about 5 minutes. Add the garlic and cook for 2 minutes more. Add the tomatoes and capers, season with salt and pepper and cook for 5 more minutes.
3. Spoon a few tablespoons of sauce into a baking dish. Season the halibut fillets with salt and pepper and place them in a baking dish. Pour the rest of the sauce over the fish and bake until the fish is just cooked through, about 12 to 15 minutes, depending on the thickness of the fish.