Prepare and cook the mussels as directed. Shell, reserving liquor, and chop coarsely. Melt butter in a saucepan; add flour and stir until smooth. Add hot milk and stir over low heat until thick. Remove from heat. Add grated Parmesan and season to taste with salt and freshly ground black pepper. Stir in chopped mussels and mussel liquor, and allow mixture to cool slightly.