Preheat oven to 425 degrees F.
In the bowl of a food processor or blender, finely grind the walnuts with the sugar to make a paste. Place in a small bowl and mix well with the egg.
On a lightly floured surface unfold the pastry and roll into a rectangle about 14 by 9-inches. Transfer to a buttered large baking sheet and spread the walnut paste evenly over the bottom, leaving a 1/4-inch border. Fold the dough border over to form an edge. Arrange the apricot and peach slices in overlapping rows on top of the paste and sprinkle with the sugar.
Bake until edges are just golden brown, about 20 minutes. Transfer to a wire rack.
In a small saucepan combine the jam and Cognac and bring to a simmer, stirring, just until the jam is melted. Brush the hot galette with the jam and let sit for 5 minutes before serving.
To make the crème frâiche garnish, beat together the crème frâiche with the sugar. Serve with the galette.
Yield: 6 to 8 servings