Melt the butter in a large saucepan over medium-low heat. Stir in the garlic, onion and tarragon. Cook until the onion has softened; about 5 minutes. Add white wine and cook for 5 minutes. Pour in milk and Parmesan cheese; increase heat to medium-high and bring to a simmer.
Whisk the flour into the water; dissolving any lumps. When the sauce has come to a boil stir in the flour mixture to thicken to desired consistency. Season to taste with salt and pepper.
Im not that impressed with this recipe to be honest. I followed it letter for letter but found the sauce to be very bland and tasteless so i added a little chilli flakes and some italian herbs and it did improve it alot - 05 Jun 2010