Preheat oven to 350 degrees F. Heat oil in a medium skillet over medium heat. Add onion and garlic, and cook until lightly colored, about 3 minutes. Add tomato, jalapeno and white wine. Cook until wine is reduced by 2/3, about 5 to 7 minutes. While sauce is reducing, heat butter in a separate skillet over medium heat. Season fish with salt and pepper. When butter is melted, add halibut to the pan. Cook for 2 minutes, flip over and then transfer pan to preheated oven and bake until cooked through, about 10 minutes. Remove sauce from heat when reduced and stir in capers and caper juice.