uhm....yuck.
Here's what you do.
Chop up those onions, heat up a bit of olive oil on high heat in a pan and toss the onions in, stir/toss to coat them with the oil. Reduce heat a little bit.
Let them cook a bit, then add just a little bit of salt. Stir from time to time and when they start to brown bring the heat back up and add some sugar. Keep cooking until they are carmelized, nice and brown.
That takes care of your onions.
You could melt the cheese with the onions....but you really need some meat of some sort to finish off that sandwich.