To thicken a white wine sauce for fish, you can use a roux, which is a mixture of equal parts flour and butter. First, I melt the butter in a saucepan and then gradually whisk in the flour (gluten free flour in my case!). I cook the roux over low heat until it's golden in color, making sure to stir it constantly to avoid burning. Once the roux has thickened, I slowly pour in the white wine while continuing to whisk. This will help create a smooth, thickened white wine sauce that is perfect for pairing with fish dishes.