Sift salt, baking powder and flour together; rub in fat and add cold water quickly, mixing lightly. Gather mixture up into a ball and place on large sheet of greaseproof paper. Put another sheet on top and roll pastry through this to required size. Transfer to flan dish and line. Prick base with a fork and prepare filling.
Preheat oven to 200 C / Gas 6.
Cook mushrooms and onion in oil until tender.
Heat milk and cream to just below boiling point. Combine with eggs. Season with nutmeg, salt and pepper. Mix with vegetables and pour in pastry case.
Sprinkle grated cheese on top and bake in centre of oven for 10 minutes. Reduce temperature to 180 C / Gas 4 and bake until filling has set (about 35 minutes).
You can use margarine or butter, or a combination, instead of the vegetable fat in the pastry. For best results, ensure all ingredients are cold and handle very lightly and minimally.