Preheat oven to 350 degrees F.
In a medium saucepan, melt the butter over medium-high heat. Add the flour and stir continuously, until the mixture is smooth and forms a ball. Add the shallots and garlic, and cook, stirring constantly for 1 minute, or until the vegetables are softened. Add the wine and the cream and whisk until the mixture is smooth and slightly thickened. Remove the sauce from the heat and add the basil leaves.
Season the fillets with salt and freshly ground pepper and place them in a lightly greased 9 by 13-inch pan. Pour the basil sauce over the fish and sprinkle with diced tomatoes. Cover the pan with aluminum foil and bake about 15 minutes or until fillets are cooked through and flake easily.
Remove the fillets from the sauce to a plate, and cover to keep warm. Remove and discard the basil leaves from the cream sauce and pour the cream and tomatoes into a small saucepan. Bring the cream sauce to a boil over medium-high heat and reduce for 5 minutes, or until the sauce is slightly thickened. Adjust the seasonings with salt and freshly ground black pepper. Add 1/4 cup of chiffonade basil and remove the sauce from the heat.
Place the fillets on individual plates and top immediately with the basil sauce and tomatoes. Garnish with fresh chiffonade of basil if desired.
Yield: 4 servings