In a small skillet, cook sausage over medium heat until no longer pink; drain and set aside. In a large saucepan, melt butter over medium heat. Stir in flour and salt; gradually stir in milk. Bring to a boil; cook and stir for 2 minutes or until thickened. Reduce heat; stir in cheese until melted.
Remove from the heat. Stir a small amount of hot mixture into the egg yolks; return all to the pan, stirring constantly. Stir in the sausage. In a small bowl, beat egg whites on high speed until stiff peaks form. Fold into sausage-cheese mixture.
Transfer to an ungreased 2-qt. baking dish. Place in a large baking pan. Add 1 in. of hot water to larger pan. Bake, uncovered, at 325° for 45-50 minutes or until a knife inserted in the center comes out clean.