Make the pastry: Combine flour, powdered sugar, and salt in a food processor. Add cold butter and pulse until coarse crumbs form. Add egg yolk and ice water, pulsing until dough just comes together. Shape into a disc, wrap in plastic, and chill for 1 hour.
Roll and fit the dough: On a floured surface, roll dough into an 11-inch circle. Press into a 9-inch tart pan, trim excess, and prick bottom with a fork. Chill for 20-30 minutes.
Blind bake: Preheat oven to 375°F. Line the chilled shell with parchment and fill with pie weights. Bake for 15 minutes, remove weights, and bake 10-12 minutes more until pale golden. Cool completely.
Make pastry cream: Heat milk with half the sugar until steaming. Whisk egg yolks with remaining sugar and cornstarch until smooth. Temper the eggs by slowly adding hot milk while whisking. Return to heat and cook, stirring constantly, until thickened. Remove from heat, stir in butter and vanilla, then strain and chill with plastic wrap on the surface.
Prepare the fruit: Wash and dry peaches and strawberries. Slice peaches into ¼-inch thick pieces. Hull and halve strawberries. Pat fruit dry if very juicy.
Assemble: Whisk chilled pastry cream until smooth. Spread evenly into the cooled tart shell. Arrange peach slices in overlapping concentric circles, filling gaps with strawberry halves.
Glaze: Warm apricot jam with water until melted and smooth. Brush glaze gently over the fruit. Let set for 10-15 minutes before serving.