Meanwhile, whisk milk, mustard, flour, salt and pepper in a medium bowl. Heat oil in a medium saucepan over medium-high heat. Add garlic and cook, stirring, until fragrant and lightly browned, 30 seconds to 1 minute. Whisk in the milk mixture. Bring to a simmer, stirring constantly, and cook until thickened, 1 to 2 minutes. Stir in tarragon, lemon zest and juice.