Peel your onions and preferably using a mandoline cut them all thin
In a sauce pan, add butter, and cook onions until soft, and slightly brown. For deeper flavor you can allow them to caramelize. This is a matter of preference.
Add beef broth, balsamic, vinegar, and thyme.
Season with a pinch of salt.
Allow to simmer for 30 minutes.
If serving in a bread bowl, cut off the top of the bread and empty the insides. Drizzle olive oil on the inside and bake for 5 minutes.
Add two ladles worth of soup per piece of bread and top with a slice of the cut off bread and a slice of provolone cheese.
Bake until cheese melts, and garnish with rosemary.
Serve.