Sieve flour, salt and sugar into a mixing bowl. Rub in the softened butter with the tips of the fingers until the mixture resembles fine breadcrumbs. Do this very gently and lightly or the mixture will become greasy and heavy. Roll into a ball and chill for ½ hour or more.
When ready to use, turn on to a floured board and knead or pat pastry lightly into a round. Place in a 10-inch pie tin and press out with fingertips to line pie tin (no rolling is necessary). Finish edge of pastry as usual and prick bottom with a fork to avoid air bubbles while cooking.
Chop onions finely; sauté in butter until transparent. Cool; add flour, eggs, cream and milk, and mix well. Season to taste with salt, freshly ground black pepper and freshly grated nutmeg.
Pour mixture into pie shell and cook in a slow oven (325°-M2) for 30 to 40 minutes. Serve very hot.