Melt butter In a heavy-bottomed sauce pan. Add onion and mushrooms; sauté for 3-4 minutes.
Cover and simmer for another 4 minutes, then add flour, stirring constantly for 2 minutes. Slowly incorporate sherry, vegetable stock and milk while stirring. Add basil, thyme, salt and pepper.
Bring to a boil, then reduce heat and simmer for 15 minutes. Cool slightly. Pour into blender. Crumble in 3 oz. (75 g) of blue cheese; add heavy cream, and purée. (You may have to do this in two batches, depending on size of blender).
Pour into 4 bowls. Crumble the remaining blue cheese into soup, and garnish with a sprig of thyme. Makes 4 servings.