How to Make It
Melt 3 tablespoons of the butter in a large enameled cast-iron casserole. Add the sliced onions, thyme sprigs and a pinch each of salt and pepper and cook over moderate heat, stirring occasionally, until the onions soften and release their liquid, about 20 minutes. Add the sugar and cook, stirring often, until the exuded liquid evaporates and the onions are caramelized, about 30 minutes longer. Transfer the onions to a medium bowl and discard the thyme springs.
Melt 1 tablespoon of the butter in the casserole. Add the celery, garlic and herbes de Provence and cook over moderate heat, stirring, until fragrant, about 3 minutes. Raise the heat to high and add the sherry. Cook, stirring up the browned bits from the bottom of the casserole, until the sherry has reduced by half, about 3 minutes. Return the onions to the casserole, add the chicken stock and simmer over low heat for 30 minutes.
Preheat the oven to 400°. Melt the remaining 2 tablespoons of butter in a small pan. Spread the bread cubes on a rimmed baking sheet and toss with the melted butter. Sprinkle with the Parmesan and toss again. Bake the croutons for about 8 minutes, or until golden brown and crisp.
Add the heavy cream to the soup and simmer over low heat for 10 minutes. Working in batches, puree the soup in a food processor or blender until smooth. Return the soup to the casserole and reheat gently. Season with salt and pepper. Ladle the soup into bowls, garnish with the cheese croutons and serve.