Melt the butter in a large frying pan. When foaming, add the garlic and mushrooms and cook over a high heat for 5 minutes, stirring occasionally, until the mushrooms are lightly browned.
Stir in the marsala or sherry and simmer for 2 minutes. Add the mascarpone and stir until melted into the sauce, then stir in the tarragon. Season to taste and serve immediately with crusty bread.