This method isn't low-calorie or low-cholesterol, but it's a pretty standard method and makes a good tuna melt:
Make them in the skillet, using either wheat bread, white bread, country white bread, or sliced Italian bread.
Make a sandwich using either plain tuna or a basic tuna salad.
Add a slice of Swiss, American, or Cheddar cheese.
Optional: Add some onions, tomato slices, celery, and/or dill cubes.
For even toasting, melt a little margarine or butter in the pan, and toast one side. If necessary add a little more margarine or butter before toasting the other side. A standard "pat" of margarine/butter will make a fairly greasy sandwich, adjust how much you use accordingly. Just melt the butter enough that it will coat the side you're going to toast evenly. Adding the margarine/butter in small pieces, rather than a single pat, helps it coat the bread evenly without having to leave a larger pat so long it starts to burn.
For a cheese-toast kind of exterior, shake in some Parmesan cheese while the butter/margarine is melting before placing each side of the sandwich on the pan for toasting. (Shake a little more Parmesan cheese before turning the sandwich to toast the second side.)