Wash the leeks and slice into rings.
Bring the cream to the boil. Season with a little salt, pepper and nutmeg, add the leeks and simmer for 2-3 minutes.
Drain the leeks through a sieve and collect the cream in a bowl.
Preheat the oven to 180°C (160°C fan, gas 4). Grease the quiche tin. For the dough, beat 150g butter with a pinch of salt until creamy. Gradually beat in the eggs. Add the flour mixed with the baking powder and mix everything to a smooth dough.
Press into the tin until smooth and even then bake in the oven for 20 minutes.
Meanwhile, , clean the mushrooms and cut into slices. Squeeze the juice of half the lemon.
Saute the mushrooms in a hot pan with the remaining butter until golden.
Pour in the leek cream and simmer for a few minutes until creamy. Season with lemon juice, salt and white pepper.
Remove the base from the oven and pour in the leeks. Spread with the sour cream and return to the oven for 15-20 minutes until golden brown. Serve with the mushroom sauce.