Melt butter in a saucepan over low heat; add flour, stirring until smooth. Cook 1 minute, stirring constantly. Gradually add milk; cook over medium heat, stirring constantly, until thickened and bubbly. Stir in salt and pepper. Fold in tuna and mushrooms.
Spoon tuna mixture into a lightly greased 1-quart casserole; top with potato chips. Bake, uncovered, at 375° for 30 minutes. Serve hot.