How to Make It
In a food processor, combine the flour, hazelnuts, cinnamon, cloves, and allspice and process until the nuts are finely ground. Add the butter, egg yolks and lemon zest and pulse until the mixture resembles coarse meal. Add the ice water and pulse just until the dough comes together. Scrape the dough onto a lightly floured work surface and knead briefly. Pat the dough into a disk, wrap in plastic and refrigerate until firm, about 1 hour.
Preheat the oven to 350°. Cut off a third of the dough and keep it refrigerated. On a lightly floured surface, roll out the remaining dough to a 13-inch round. Fold the dough in half and transfer it to an 11 1/2 -inch tart pan with a removable bottom. Unfold the dough and press it into the pan and up the side as evenly as possible. Refrigerate the shell.
On a lightly floured surface, roll out the remaining piece of dough to a 13-by-8-inch rectangle. Place the rectangle on a baking sheet and freeze until very firm. Using a pizza cutter or a sharp knife, cut the rectangle lengthwise into 10 strips, cutting directly on the pan. Refreeze the strips until needed.
In a food processor, combine 1/4 cup of the sugar with the hazelnuts and ginger and process to a coarse powder. In a large bowl, toss the pears with the remaining 1/2 cup of sugar and the flour. Remove the tart shell from the refrigerator. Spread the hazelnut and ginger mixture over the bottom of the shell and top with the pears. Arrange the frozen dough strips over the filling in a lattice pattern.
Bake the tart in the center of the oven for about 1 1/2 hours, or until the pastry is brown and the filling is bubbling. Let cool on a rack for at least 10 minutes.
Meanwhile, in a large bowl, whip the cream until soft peaks form. Add the confectioners' sugar and thyme and beat until firm; do not overbeat. Serve the tart warm, with the thyme whipped cream.