Preheat the oven to 150° C (gas 1).
Wipe the large mushrooms with a damp cloth, remove the stems, rub the caps with a little lemon juice to keep them white.
Wipe and chop the small mushrooms, mix with the remaining lemon juice. Heat the butter in a small skillet.
Saute the shallots and garlic, stirring, for 4 minutes. Add the small mushrooms and wine. Cook, stirring, for an additional 4 minutes. Remove from heat. Stir in Parmesan cheese, breadcrumbs, egg, heavy cream and tarragon.
Place the mushrooms on a lightly oiled baking sheet, spread the filling on top.
Bake for 12 minutes.
Serve warm or cold sprinkled with Parmesan and parsley.