- In a large non-stick pan, sauté the onions in the oil over medium heat for about 30 minutes or until well caramelized. Add oil, if needed. Season with salt and pepper.
- Deglaze with the sherry. Add the broth, thyme, Worcestershire sauce, and vinegar. Bring to a boil and simmer gently for about 15 minutes. Remove the thyme sprigs. Add broth, if needed. Adjust the seasoning.
- Just before serving, with the rack in the highest position, preheat the oven’s broiler.
- Cover the bread slices with the fresh cheese and sprinkle with the Gruyère cheese. Lightly brown the cheese in the oven. Top with the arugula. Serve with the soup.