Preheat oven to 350 degrees F. Remove skin and bones from the fish and divide it into flakes. Set aside.
Melt butter, add flour, milk, lemon juice and salt and pepper (to taste) to make a sauce. Set aside.
Mash the yams with the 1 tbs milk and place half in the bottom of a baking dish. Place the fish on the layer of mashed yams, and cover with sauce. Cover with remaining yams.