For the shortcrust pastry:
1) Preheat the oven to 190C/Gas 5. Place the flour, butter, and sour cream in food processor and pulse to combine.
2) When the dough has formed into a ball, pat with lightly floured hands into the bottom and sides of an ungreased 25cm round tart pan, 1cm deep and with a removable bottom, or a round au gratin dish.
3) Bake for about 15 mins, until the pastry case is set but not browned. Allow to cool while preparing the filling.
For the filling:
1) Lower the oven temperature to 180C/Gas 4.
2) If using fresh peaches, peel and thickly slice the peaches. If using tinned peaches, drain well.
3) Arrange the fresh or tinned peach slices in overlapping circles over the pastry case, until it is completely covered. Overfill, because the peaches will draw up during cooking.
4) Combine the egg yolks, sour cream, sugar, and flour and beat until smooth. Pour the custard mixture over the peaches.
5) Place the tart pan on a baking sheet and bake for about 1 hr, until the custard sets and is pale golden in colour. Cover with an aluminium foil tent if the crust gets too dark. Transfer to a wire rack to cool. When cool, remove the tart pan.
For the glaze:
1) Combine the preserves or jam and lemonade. Spread with a pastry brush over the top of the warm tart.
2) Serve the tart warm, at room temperature, or chilled.
Note: If using fresh peaches, sprinkle with a little lemon juice after slicing.