Mezcal-spiked onion soup with Asiago

My Italian friend taught me how to make a simple onion soup.  We used beer (Sierra Nevada Pale Ale) because we didn't have white wine, and the result was delicious with no detectable beer taste.  This time, I had neither, so I used mezcal, as I'd just seen an article in LA Weekly about fancy and interesting new cocktails.  The picture includes a brand in my cupboard, and I love onion soup, so I went for it.  I used 12 white or sweet onions, though my friend recommends cippolini as the best-tasting.

I put them in a giant pasta pot with plenty of olive oil and a bit of good vegetable broth, cooking them down on medium heat until everything was soft and releasing water, about 30 minutes.

Then I added about 1/2 cup of mezcal and a little boiling water.  I cooked this down on high heat for quite a while, stirring occasionally, but in the end I had more liquid than when my friend made it.  The mezcal left a unique smoky flavor, very nice with a little butter, fresh ground pepper, and grated Asiago.  You could use Parmesan.