How to Make It
In a medium bowl, cover the dried porcini mushrooms with the hot water and let them stand until softened, about 20 minutes. Strain and reserve the porcini soaking liquid.
Meanwhile, in a large enameled cast-iron casserole or soup pot, heat the olive oil. Add the onion and cook over moderate heat, stirring occasionally, until softened, about 5 minutes. Add the white mushrooms, brown sugar and thyme, cover and cook over moderately high heat until the mushrooms soften, about 3 minutes. Add the wine and cook over high heat until it has reduced to 1/3 cup, about 5 minutes.
In a medium saucepan, melt the butter over moderate heat. Whisk in the flour and cook for 3 minutes, whisking. Whisk in the half-and-half and bring to a simmer over moderate heat; whisk constantly until thickened, about 2 minutes. Stir the half-and-half mixture into the white mushrooms, along with the porcini and their strained soaking liquid. Working in batches, puree the mushroom mixture until smooth, adding some of the chicken stock if necessary.
Return the soup to the casserole. Add the remaining chicken stock and simmer over moderate heat for 5 minutes. Stir in the heavy cream and sherry and heat through. Season the soup with salt and pepper and ladle into bowls. Top with the croutons and serve.