Clean mushrooms, remove stalks and chop them finely. Heat oil in a large frying pan, add onion and sauté on medium heat for two minutes. Add garlic and continue to sauté for another minute.
Add mutton mince and continue to cook over medium heat for three to four minutes, stirring occasionally. Add chopped mushroom stalks, crushed peppercorns and salt to taste and continue to cook over high heat for two minutes.
Remove from heat and transfer into a large bowl. When cool, add parsley, half of the grated cheese and egg and stir until well combined. Preheat oven to 160ºC.
Stuff the above mince mixture into each mushroom, sprinkle grated cheese and bake for ten to fifteen minutes in the preheated oven at 160ºC or until light golden. Serve hot.