On the stovetop, melt butter and blend in flour. Add milk, reduce heat and simmer, stirring frequently until mixture thickens. Add salt and pepper. Add Parmesan cheese and 1 cup of the cheddar cheese. In a separate pot, boil fish in water for 8 minutes or until fish starts to flake apart; remove from heat and drain off water. In another pot, boil potatoes for 5 minutes; potatoes should still be somewhat hard. Mix the contents of all three pots together, spread in a 9" x 11" dish and cover with remaining cheese. Bake at 375F for 25-35 minutes.