CHOP mushrooms. Add water to mushroom liquid to make 2 cups.
MELT butter in large saucepan over medium heat. Add onion; cook for 1 to 2 minutes or until tender. Remove from heat. Stir in flour and seasoned salt; return to heat. Stir in mushroom liquid mixture, evaporated milk and mushrooms. Cook over medium heat, stirring constantly, until mixture comes to a boil.