Preheat oven to 350 degrees.
Place fish in greased 9x13 inch baking dish.
Melt butter in a saucepan.
Add flour and salt; stir until blended.
Slowly add milk and bring to a boil, stirring constantly until the mixture thickens.
Remove from heat; blend in cheese and lemon juice.
Pour sauce over fish and sprinkle with bread crumbs.
Bake for 30 minutes