In a bowl, dissolve the powdered fish stock in the water. Set aside.
In a large saucepan, soften the onion and garlic in the butter. Season with salt and pepper. Add the flour and cook for 1 to 2 minutes, stirring constantly. Add the wine, cream, and broth and bring to a boil, stirring with a whisk.
Simmer gently, stirring frequently, for about 10 minutes or until the sauce thickens. Adjust the seasoning.