Lemon Tarragon Cream Sauce

Lemon Tarragon Cream Sauce
1/4 cup butter
1/4 cup all-purpose (plain) flour
1 cup milk
2 cups whipping cream (35%)
1 onion, chopped
1 Tablespoon garlic, minced
2 Tablespoons chopped tarragon
zest of one lemon
1/4 cup Parmesan cheese
salt, to taste
pepper, to taste
1/4 cup butter, cold and cubed

In a medium saucepan over medium heat, melt the butter. Add the flour and whisk until smooth. Gradually whisk in the milk until smooth. Whisk in the cream. Stir in the onion, garlic, tarragon, and zest and bring mixture to a boil. Stir in the cheese, then season to taste. Gradually whisk in the butter, cube by cube. 

This sauce is a basic cream sauce recipe started with a thick bechamel sauce. It is extremely forgiving and versatile. Here are some variations on preparation for basically the same sauce with slightly different tastes or textures.

1) After the flour is mixed in continue cooking, whisking occasionally until the butter flour mixture turns light brown. This forms a brown roux which will give the sauce a richer flavor and darker color. Then add the milk and proceed as directed.
2) Add the onion and garlic initially with the butter to give them more color and texture. Cook them to the desired degree of caramelization just be careful not to burn the garlic.
3) Add the onion and garlic only with the cream then strain out after the mixture boils. This will add their flavor but eliminate the texture of them. Then add the tarragon and lemon and proceed.
4) Puree the sauce at any point for a smoother texture. 
5) Bring the mixture to a boil after adding the milk or boil it with the cream longer for a thicker sauce.
6) Add more milk for a thinner sauce.