Season the halibut with salt and pepper. In a medium saute pan over high heat add the oil. Once heated, add the fish and sear until golden brown. Add butter, thyme, and garlic and baste. Set aside.
In a small saucepot combine the butter, fish fume, diced potatoes and simmer over medium heat. Add fennel, tomatoes, olives and basil. Season with salt and pepper, to taste, and add a dash of sherry vinegar. Add the roasted peppers and gently warm. Adjust seasoning, if necessary.
In a large bowl place the olive ragout and garnish with the roasted peppers and parsley oil. Place the halibut on top and finish with the scallion.