Trim the mushrooms, disregarding the tough ends of the stems. Wipe with a damp cloth then slice.
In a heavy bottomed saucepan, melt the butter and cook and stir the mushrooms. Add a little water so they don't stick to the bottom and simmer over a low heat for 1 hour, stirring occasionally.
Add the chopped onion and simmer for 15 more minutes, stirring occasionally. Remove from the heat and allow to cool. In a separate bowl, whisk 2 to 3 tablespoons of the sauce with the cream and slowly pour back to the saucepan, whisking continuously.
Heat the sauce through and season to taste. Garnish with freshly chopped parsley.