Heat the butter over low heat in a large pot until melted. Add the onions and caramelize them very well while occasionally stirring, which can take up to an hour.
Halfway through the cooking process, you can add in some sugar to help brown and add more sweetness. This will be heavily scrutinized. Sugar makes caramel, so it’s kind of the same thing in this soup.
Once caramelized, add in the garlic and sweat for 3 to 4 minutes. Next, de-glaze with red wine and cook the liquid until it is mostly absorbed into the onions. This process is known as “au sec.”