Melt butter in a 3-quart pan, and sauté onions until translucent. Add portabellas and cook for 4 to 5 minutes, until soft. Add tarragon and vermouth and boil for 2 minutes. Lower heat and stir in flour until smooth. Stir in heavy cream and bring to a simmer. (Do not boil, or cream will overflow.) Simmer for 5 to 10 minutes until soup begins to thicken, stirring every few minutes to prevent burning. Add Gorgonzola and stir until smooth. Add salt and pepper to taste.
* For a low-fat version, substitute half-and-half or milk for heavy cream, but increase amount of flour.Makes 4 Source: