How to Make It
In a food processor, combine the flour with the salt, butter and shortening and process for 5 seconds. Sprinkle the ice water over the flour mixture and process until the dough just begins to come together, about 10 seconds; you should still be able to see small pieces of butter in it. Turn the dough out onto a work surface, gather any crumbs and pat it into a disk. Wrap in plastic and refrigerate until chilled, about 30 minutes.
Preheat the oven to 400°. On a lightly floured work surface, roll out the dough to an 11-inch round about 1/8 inch thick. Fit the pastry into a 9-inch fluted tart pan with a removable bottom; trim the overhanging pastry. Patch any cracks with the pastry trimmings. Using a fork, prick the bottom of the tart shell several times to allow steam to escape during baking. Refrigerate the tart shell for 30 minutes, or until chilled.
Line the tart shell with foil and fill with pie weights or dried beans. Bake for about 17 minutes, until lightly golden around the rim. Remove the foil and weights and lower the oven temperature to 375°. Bake for about 13 minutes longer, until the pastry is lightly golden all over. Let cool completely on a rack. Leave the oven on.
In a large skillet, cook the bacon over moderately low heat, stirring frequently, until crisp, about 8 minutes. Using a slotted spoon, transfer the bacon to a paper towel–lined plate. Add the onion to the skillet and cook over moderate heat, stirring, until softened, about 1 minute. Reduce the heat to low, add the maple syrup and cook until the onion is lightly caramelized, about 5 minutes.
In a blender, combine the goat cheese, sour cream, egg, egg yolk, allspice, salt and pepper and blend until smooth. Add the heavy cream and pulse just until the custard is blended.
Scatter the onions, bacon and Gruyère cheese evenly in the cooled tart shell. Pour in the custard, being careful not to overfill; you may have a bit of custard left over. Bake the tart in the center of the oven until golden brown and the center is just set, about 25 minutes. Cool on a rack for 15 minutes, then carefully remove the tart ring and serve warm.
The baked tart shell can be stored at room temperature for 2 days.