1. Spread sausage in ungreased pie plate, 9 x 1 1/4 inches. Cover tightly and microwave on high 3 to 4 minutes or until no longer pink; crumble. Drain sausage; drain fat from pie plate.
2. Arrange half of the croutons in same pie plate. Layer with cheese, sausage, onions and remaining croutons. Beat milk, mustard and eggs; pour evenly over croutons. Cover and refrigerate at least 2 hours but no longer than 24 hours.
3. Elevate pie plate on inverted dinner plate in microwave oven. Microwave uncovered on high 2 to 5 minutes.
4. Microwave uncovered on medium (50%) 18 to 24 minutes, rotating dish 1/2 turn after 10 minutes, until knife inserted in center comes out clean.