From "French Women Don't Get Fat" by Mireille Guiliano. Page 104 of the soft-cover book (that we own) states that this recipe serves 10. It is possible that the other reviewer has a different book, or that one of the books has a typo.
FOR THE PIECRUST: Put the flour, salt, and sugar in a food processor and pulse to combine. cut the chilled butter into small pieces and add to the processor with the water for 15 seconds. If the dough is too dry, add more water by droplets. The dough should just hold together. Do not knead. Wrap the dough in waxed paper and refrigerate at least 4 hours to overnight.
Preheat oven to 400°F Roll out the pastry and fit to a 9 inch pie plate, 2 inches deep. Prick the bottom and sides with a fork then line the plate with aluminum foil and add dry beans to weigh down the dough. Bake for 10 minutes. Remove the foil and beans.
FOR THE FILLING: Increase oven temperature to 450°F In a bowl mix hazelnuts, 1/4 cup brown sugar, and butter. Work into a paste and spread it into the pie shell. Bake 10 minutes.
FOR THE TOPPING: Meanwhile, Mix the remaining ingredients in a large bowl. Pour into the pastry shell. Reduce oven temperature to 325°F Bake pie for 45 minutes. Serve at room temperature or cold.