Roll up crepes. In a 13 x 9-inch baking dish, place crepes seam-side down. Preheat oven to 375 F. In a medium saucepan, melt margarine; blend in flour, 1/2 tsp tarragon, salt & pepper. Whisk in half & half, stirring constantly over medium-high heat until mixture thickens & bubbles. Stir in sliced mushrooms. Pour sauce over crepes in baking dish. Bake 25 minutes or until heated through.