The article references ground almonds but the recipe doesn’t include them. I love almonds and so am interested in how much you would use.

You can add almond flour whisked into the broth to thicken it, i would start with 1/4-1/3 cup and taste to see how you like it, adjust with more broth if needed.
A quick search i also found this other recipe, very different flavor profile since it includes cinnamon and will be a bit sweet/sour as a finished soup
https://memoriediangelina.com/2014/01/12/carabaccia-tuscan-onion-soup/