In large skillet over medium heat, melt butter. Add onions, salt and pepper, stirring occasionally until soft, about 5 minutes. Reduce heat to very low, stirring occasionally, until onions are pale gold, about 20 minutes. Set aside to cool.

On a lightly floured surface, roll out dough to a 14-inch circle. Fit dough into an 11-inch fluted pan with removable bottom. Trim excess to a 1/2-inch overhang, then fold it back into pan to reinforce fluted edge. Using a fork, pierce bottom of pastry all over. Refrigerate for 30 minutes.

Preheat oven to 400°F.

Place chilled shell on rimmed baking sheet and line shell with foil and pie weights. Bake until pastry is set and pale gold along the rim, 15 to 20 minutes. Carefully remove foil and weights and continue baking until shell is golden all over, 10 to 15 minutes.

Meanwhile, in medium bowl, whisk together eggs, cream and nutmeg. Stir in onions. Peel, core and thinly slice apple.

Remove shell from oven, pour egg mixture in, and arrange apple slices decoratively on top. Press apples to slightly submerge them. Bake until top is lightly browned, 25 to 35 minutes. Let stand for 10 minutes before slicing and serving.