Place in rough mounds over the mushroom sauce (don't be tempted to smooth it), then bake for 45 minutes at 200C/gas mark 6. Eat as a meal or serve as a warm accompaniment with your braai or roast dinner. Cook topping Preheat oven to high grill. Fry the leeks and garlic for 2 minutes, to soften. Add the olive oil to a large saucepan on a medium heat. Top with the cheese. Instructions. With a cream and Marsala sauce and a crunchy potato topping, a leek and mushroom pie will have you licking the bowl clean Nigel Slater Sat â¦ Bake for +- 25 minutes. Cook unpeeled potatoes in boiling, salted water until tender, then mash roughly, skins and all, with butter. Add the mushrooms, herbs, salt and pepper and stir for a minute. Cook on high heat for 15-20 minutes, until tender. the potatoes and place a layer into a baking dish, season well and pour half of the sauce onto the potatoes, cover with another layer of seasoned potatoes followed by the remaining sauce. Peel and dice potatoes 2cm. 5. Mash until smooth and season. Drain and return to pot with milk, 1/4 tsp salt and a knob of butter. Add potatoes to a pot and cover with hot tap water.