1. Preheat oven to 180ºC .
2. Place half of the mashed potatoes in a buttered 23 cm pie plate.
3. Saute the mushrooms and onion in the butter. Add the lemon juice, salt and pepper and spoon over the potato layer. Spread with sour cream. Top with remaining potatoes.
4. Bake for 45 minutes, until brown. To serve, cut into wedges.