Instructions: 

1. In a saucepan, cook onion in butter over low heat, stirring, until it is softened. Stir in flour and cook, while stirring, for 3 minutes.
2. Add milk in a stream, whisking vigorously until the mixture is thick and smooth. Add white wine and again whisk until fully incorporated.
3. Add salt and fresh tarragon. Simmer the sauce for 10-15 minutes, or until it is thickened to the desired consistency.
4. Meanwhile, cook ravioli in 4 quarts of water for each pound. Add a hearty pinch of salt. Bring water to a full boil, reduce heat and then add ravioli. (If you add ravioli when the water is at a robust boil, you risk having the ravioli split open.) Occasionally, stir gently to prevent ravioli from sticking to the bottom of the pot. Once water returns to a gentle boil, cook about 8-12 minutes. Drain carefully, or simply use a slotted spoon to remove the ravioli individually from the pot.
5. Serve over ravioli.