- Spread the toasted bagel halves with butter or mayonnaise.
- Arrange the bagel halves, cut side up, on a toaster oven tray.
- Layer the prosciutto slices on top of one bagel half; top with onion and cheese slices Slide the cheese topped bagel half into a toaster oven, and heat on the top brown or toast setting until cheese melts.
- (Or place the bagel half on a broiler pan under preheated broiler, 4" from the heat suorce, for about 1-1 1/2 minutes, or just until cheese melts) Top with second bagel half.
- Serve the sandwich immediately with deli mustard.