Heat the olive oil in a large pot over medium heat. Add the onions and sugar and cook, stirring occasionally, until the onions are deeply caramelized, about 30-40 minutes. Be patient, as this is the key to the soup's flavor.

Pour in the white wine and cook for 2-3 minutes, scraping up any browned bits from the bottom of the pot.

Add the beef broth and bay leaf. Bring to a boil, then reduce heat and simmer for 20-30 minutes.

Remove the bay leaf. Season with salt and pepper to taste.

Stir in the parsley and lemon juice just before serving.