In Germany, this dish is typically served at harvest time and washed down with a glass of Federweisser (a new wine that hasn't finished fermenting). Barring that, a full-bodied dry Rheinpfalz Riesling is recommended as an accompanyment.
Recipe: The New German Cookbook
In a heavy 12-inch skillet saute the bacon over low heat for about 5 minutes, until all the drippings cook out and only crisp browned bits remain. Using a slotted spoon, lift the bacon bits to paper toweling to drain and reserve. Pour off all but 1 Tbsp. drippings from the skillet
Add the onions to the skillet, sprinkle with caraway seeds and nutmeg, and saute 5 minutes over moderate heat, stirring often. Reduce heat to low, cover the skillet, and steam the onions for 15 minutes. Remove from heat and set aside.
Roll the puff pastry on a floured surface into a 14-inch circle. Set a 12-inch tart tin on a baking sheet. Lop the pastry over the rolling pin and ease into the tin, pressing it in to form a neat pastry shell. Trim the pastry overhang to 1/2-inch larger all around than the tart tin, then roll under so it rests on top of the tin and crimp, making a high fluted edge.
Pile the onion mixture into the pastry shell, spreading to the edges as smoothly as possible, and then scatter the reserved bacon evenly on top.
Bake, uncovered, in a preheated 350-degree oven for 20 minutes. Meanwhile, whisk the cream, eggs, salt and pepper until well blended and set aside.
Remove the partially baked onion cake from the oven and raise the heat to 400 degrees.
Slowly pour the cream mixture over the onion and bacon, distributing it as evenly as possible. Return cake to the oven and bake, uncovered, 20 to 25 minutes, until the filling is set like custard and lightly browned. Remove the onion cake from the oven and cool 30 minutes.
To serve, cut into slim wedges (for appetizer portions) or medium-size wedges (for a main course).