Confectioners' sugar for finishing FOR THE DOUGH: COMBINE DRY INGREDIENTS in bowl of food processor and pulse several times to mix. Cut butter into 8 pieces and distribute evenly over dry ingredients in work bowl. Pulse until very finely powdered. Add egg and continue to pulse until dough forms a ball that revolves on blade. Remove dough, press into a disk, wrap and chill. TO POACH THE PEACHES: CUT A CROSS IN THE BLOSSOM end of each peach and immerse in boiling water about 30 seconds. Remove peaches to a bowl of ice water and slip off skins after they cool. Cut each peach into eight wedges, cutting them away from the pit as they are removed. Arrange slices in a wide, shallow non-reactive pan in one layer and sprinkle with sugar. Cover and place over low heat and bring to a simmer. Remove from heat. Cool. Drain peaches gently by lifting them from the juices with a slotted spoon to a plate or other non-reactive surface. Reserve the juices for another use. SOUR CREAM FILLING: PREHEAT THE OVEN TO 350F and set a rack in the lower third. Roll the pastry dough on a floured surface and line a 10-inch tart pan with it. Arrange the peach wedges on their sides in a concentric pattern. Whisk the filling ingredients together, strain and skim foam from top. Pour filling over peaches and sprinkle evenly with the sliced almonds. Bake the tart about 40 minutes, until the crust is baked through and the filling has set. Cool on a rack. Unmold, and serve at room temperature. Dust with the confectioners' sugar immediately before serving. Store leftovers refrigerated.