Most recipes call for a bit of brandy and for that I like to use a French Calvados, an apple brandy from the region of Normandy. After all, the origins of this soup are definitely French so Calvados is a natural fit. It also adds a nice touch of sweetness to the onions. If you don’t have Calvados, just substitute another brandy.
As for the wine, most recipes call for a dry white wine. In a nod to this being an Italian version, I use a local red. My go to favorite everyday Italian, Urlo di Lupo from Tenuta Maria Teresa in the hills outside of Lucca, works perfectly. The red adds a depth of flavor that pairs well with the next ingredient - beef broth. There are lighter versions of onion soup that call for chicken broth. If using chicken broth I would definitely use a white wine, but I like my onion soup bold and a bit more intense so I go with red wine and beef broth. And while homemade beef stock is ideal, I am unlikely to have a stash of homemade broth in my tiny freezer so I cheat and use store bought.
Now for the most important ingredient and, for me at least, the hardest - the patience required to caramelize all those onions. There is simply no way to speed up this process. I have tried (and failed) many times. Start with melted butter and thinly sliced onions in a large pan over low heat, stirring until the onions soften and start to become translucent. Then, nudge the heat up a bit towards medium and back off! If stirred too often the onions never rest on the bottom on the pan long enough to caramelize and end up as just a pile of soft translucent onions. The same can happen if using too small a pan, so give those onions plenty of room to contact the bottom of the pan. Cook for 5 or 6 minutes and then stir. Keep repeating this step, moving a bit of onion to the side of the pan every 5 minutes or so to see that there is some brown on the surface of the pan and the onions are sticking a little to the bottom (but not burning or getting crisp; adjust flame as needed). Then give them a stir bringing the brown onions up to the surface. If not, wait a bit longer. The process of caramelizing will take about an hour. At first it seems nothing is happening, but then the onions begin to change color and eventually become a perfect deep brown color and very fragrant.