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.
For the avocado mousse
Place avocados, cream and salt in the bowl of a food processor. Whip until light and fluffy. To serve, divide sauce equally among 4 dinner plates. Place halibut in the center of each plate on top of the sauce. Put avocado mousse into a pastry bag and pipe it onto the plate, sprinkle with lime juice and garnish lime slices and cilantro.