You will need:
A toaster oven (regular oven will suffice but is really too big)
plain or sesame seed bagels, sliced
sliced cheddar or jack cheese
On each bagel half, spread a thin layer of the mustard. Arrange a tomato slice and slice of cheese on top of each one. (If you like, you can add a slice of meat such as ham under the cheese.) Place in the toaster oven and set the timer to medium (or until cheese melts.) Eat.
Serves one person per bagel.