How to Make It
In a large deep skillet, heat 1/3 cup of olive oil. Add the onions and fennel and cook over moderately low heat, stirring occasionally, until softened, about 10 minutes. Add the garlic and cook until fragrant, about 1 minute.
Add the tomatoes to the skillet and break them up with a fork. Season with salt and pepper and cook over low heat until most of the liquid has evaporated, 15 to 20 minutes. Add the wine, stock and capers and cook over low heat for 10 minutes. Stir the chopped basil and the butter into the sauce.
Light a grill. Brush the halibut fillets with olive oil and season generously with salt and pepper. Grill over high heat until just cooked through, about 4 minutes per side. Spoon the sauce onto a large deep platter, set the fillets on the sauce and garnish with basil leaves. Serve hot or at room temperature.