Preheat oven to 150 degrees C. Lightly grease a medium baking dish.
Melt butter in a medium saucepan over medium heat, and stir in flour until smooth. Gradually stir in chicken stock and sherry. Cook, stirring constantly, until thick and bubbly. Mix in chives, basil and thyme. Remove from heat and set aside.
In the prepared baking dish, arrange the fish fillets in a single layer. Season with salt and pepper. Arrange tomato slices over the fish. Cover with the stock mixture.
Bake 30 to 40 minutes in the preheated oven, until fish is easily flaked with a fork.