Step 1 Remove the beans from the pods. Bring a large pot of water to boil, add the beans and simmer about 2 to 3 minutes until the beans are tender and the outer skins are loose. Drain and rinse under cold water.
Step 2 Pop each bean out of its shell. You will have about 2 cups of fava beans. Set the beans aside while making the bechamel sauce.
Step 3Heat the butter in a small saucepan until melted. Add the minced shallots and saute over medium-low heat until tender, about 1 minute. Add the flour and warm for a minute, stirring. Gradually add the milk and whisk until smooth. Add a sprig of thyme and bring to a simmer. Reduce heat to low and simmer 10 minutes, stirring occasionally to prevent the sauce from sticking to the bottom. Strain through a fine mesh strainer.
Step 4Stir in the lemon juice, tarragon, salt and pepper.
Step 5Stir the peeled fava beans into the cream sauce. Heat 1 to 2 minutes just until the fava beans are hot. Season to taste with salt and pepper.