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.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.