In 2-quart saucepan, slowly stir grits, and salt into boiling water. Reduce heat to medium-low, cover.
Cook 5-7 minutes, stirring occasionally. Remove from heat; stir in cheese, onions, and mustard, stirring until cheese melts. Pour grits into 9- to 10-inch pie plate, which has been lightly coated with cooking oil spray.
Cover and chill until firm. Cut into 8 pie-shaped slices. Arrange grits on a baking pan which has been lightly coated with cooking oil spray. Broil 8-10 minutes or until golden brown.