It's one of my favorites and very easy to prepare.
I melt a stick of butter in a large pot. I then cook the peeled and sliced up onion wedges until the onion "caramelizes" (it turns brown as the heat crystallizes sugar, making it sweet). This usually takes a little over an hour. Then add water for stock, with a beef bouillon cube for each cup of water you add. Slice up an entire piece of mozzarella cheese and add the slices. Cook for a few minutes until flavors blended. Add salt and pepper to taste.
A tip about peeling onions. I take off not only the skin but the outermost layer of onion too to discard. This is because pesticides and other chemical residue tends to concentrate in the outer layer of fruits and veggies. This is why it's a good idea to peel fruit and veggies, even though you are discarding good nutrition and fiber. I just don't trust what is put on produce. Taking off the outer layer gets rid of most of the risk.