Heat the butter in a large saucepan over medium heat, about 4 minutes. Add the onions with a pinch of salt and patiently cook, stirring often, until caramelized, 15-20 minutes. If the pan becomes too dry during cooking, add a splash of water (1-2 tablespoons).
Add the flour and mix with a wooden spoon for about 1 minute, until it starts to brown.
Add the wine. Mix well. Cook for about 2 minutes, until most of the wine evaporates.
Add the beef broth, black pepper, and dried thyme. Bring to a boil over high heat. Reduce the heat to medium-low and simmer, uncovered, stirring often, until the soup is noticeably thicker, about 20 minutes. Serve immediately.
This is not a gluten-free or low-carb recipe.I typically use yellow onions in this recipe, but white onions should work too. I don't recommend using red onions in this recipe.Be sure to cook the onions slowly and patiently over medium heat until they are beautifully caramelized. This is where the soup gets its rich flavor and color. Don't attempt to shorten the process by turning the heat higher than medium!