Place potatoes in a large saucepan and cover with water. Bring to a boil. Reduce heat; cover and simmer for 15-20 minutes or until tender. Cook beans according to package directions.
Meanwhile, melt butter in a saucepan; stir in flour, salt and pepper until smooth. Gradually add milk. Bring to a boil; cook and stir for 1 minute or until thickened. Drain potatoes and beans; place in a serving bowl. Add sauce and stir to coat.