In a food processor, combine flour, powdered sugar, and salt. Pulse in cold butter until mixture resembles coarse breadcrumbs with pea-sized butter pieces.
Whisk together egg yolk and heavy cream. With processor running, pour in mixture and process until dough begins to clump. Press into a disc, wrap in plastic, and refrigerate for at least 1 hour.
Roll chilled dough into a 12-inch circle about ⅛ inch thick. Gently press into a 9-inch tart pan with removable bottom, pressing into fluted edges. Trim excess dough and prick the bottom with a fork.
Place lined pan on a baking sheet and refrigerate for 30 minutes. Preheat oven to 375°F (190°C). Line the shell with parchment paper, fill with pie weights, and bake for 18-20 minutes until edges are lightly golden.
Remove parchment and weights. Return shell to oven and bake 8-10 more minutes until base is golden and dry. Cool completely in pan on a wire rack.
Heat milk in a saucepan until steaming. Whisk egg yolks and sugar until pale and ribbony. Whisk in cornstarch and flour. Temper the yolks by slowly pouring in hot milk while whisking constantly.
Return mixture to saucepan and cook over medium heat, whisking vigorously, until thickened and large bubbles appear. Cook for 1 more minute, then remove from heat. Stir in butter and vanilla.
Strain cream through a fine mesh strainer into a clean bowl. Press plastic wrap directly onto the surface and refrigerate for at least 2 hours until completely cold and set.
Peel peaches by scoring an X on the bottom, blanching in boiling water for 30-60 seconds, then plunging into ice water. Slip off skins, halve, remove pits, and slice into ¼-inch wedges. Toss with lemon juice.
Whisk chilled pastry cream to loosen. Spread evenly into the cooled tart shell, filling to about ¼ inch below the top edge. Smooth the surface flat.
Arrange peach slices in overlapping concentric circles, starting from the outside and working inward. Angle each slice slightly with the rounded side facing outward. Fill the center with a rosette of slices.
Combine apricot jam and water in a small saucepan. Heat until melted and fluid. Brush the warm glaze gently over all the peach slices in a thin, even layer.
Refrigerate the tart for at least 30 minutes to set. Remove from pan by placing on a tall can and pressing the ring downward. Slide onto a serving platter and serve cold or at room temperature.