In a Dutch oven, sauté the sliced onions in a drizzle of olive oil until they are translucent. Add the flour and stir vigorously. Pour in the vegetable broth and add the thyme. Season generously with black pepper and bring to a boil, stirring. Reduce the heat and simmer for 20-25 minutes.

Preheat the oven to broil.

Pour the soup into oven-safe bowls, add the bacon, place the toasted bread slices on top of the soup, and add a piece of Reblochon (or other cheese) on the bread. Broil for a few minutes until the top is golden and bubbly.