Preheat oven to 180° C.
Put chicken pieces in a bowl. Add salt and crushed peppercorns and mix well.
Heat 3 tablespoons oil in a non-stick pan.
Coat chicken pieces with flour and shallow-fry on high heat, turning sides, till golden brown on both sides. Transfer into an ovenproof baking dish.
Heat remaining olive oil and 1 tablespoon butter in the same pan. Add onion, garlic and celery and sauté for 1 minute.
Add mushrooms, salt and crushed peppercorns, mix and cook for 1 minute. Spread the mixture over the chicken pieces in the dish.
To make sauce, heat 1 tablespoon butter in the same pan. Add 1 tablespoon flour and sauté till light brown.
Add chicken stock and whisk well so that no lumps remaining and cook till the sauce thickens. Add salt, crushed peppercorns, lemon juice and parsley and mix well.
Add cream, mix well and pour over the mushrooms.
Mix together potatoes, remaining butter, parmesan cheese powder, salt, crushed peppercorns, thyme and oregano in a bowl. Spread this mixture over the sauce in the dish, place the dish in the preheated oven and bake for 15-20 minutes.