Fillings could be ham and cheese (swiss, muenster, chedder -), cheese and some peppers fried till they're soft, mustard spread on the bread and anything above, crumbled pre cooked bacon and anything, corned beef and anything, pastrami and anything, turky and anything, and so on. You could also make a bechamel sauce with cheese in it. I find it to be overkill.
So, it's filled. Fry on one side in a fairly dry pan. Cover until it looks like the cheese is melted inside and the bottom is lightly browned. Flip it. While the second side fries, grate some cheese.
Lift your sandwhich. Sprinkle the cheese on the pan and kind of try to have it be a sandwhich sized plop. Drop the sandwhich on the cheese. Carefully poke the sandwhich, and as the cheese browns, it will not stick. Once the cheese is attractively browned, remove, flip cheese side up, and serve.
Good with soup.