Prepare Flaky Pastry and set aside.
Melt butter in a 3 to 4 quart pan over medium heat.
Add onions and cook, covered, stirring often, until soft.
Uncover and continue cooking, stirring often, until lightly browned.
Stir in flour, salt, nutmeg, and mustard.
Remove from heat and slowly pour in milk, stirring.
Return to heat and bring to a boil, stirring; continue boiling and stirring until sauce thickens.
Set aside.
In a large bowl, beat eggs until blended, then mix in sour cream and cheese.
Stir onion mixture into sour cream mixture; season to taste with salt.
On a floured board, roll out pastry to form a circle about 12 inches in diameter; fit dough into a 10 inch pie plate and fold excess under; flute edge.
Pour in filling.
Bake on bottom rack of a 350° oven for 55 minutes or until crust is browned and filling is firm when plate is gently shaken.
Let cool on a rack for 5 to 10 minutes before cutting into wedges to serve.