To make the pastry, process the flour, icing sugar and butter in a food processor until mixture resembles fine breadcrumbs. Add egg yolks and water and process until mixture just comes together. Turn onto a lightly floured surface and gently knead until smooth. Reserve one-quarter of the pastry.
Roll out remaining pastry on a lightly floured surface to a 30cm disc. Line the base and side of a 23cm round (base measurement) fluted tart tin with removable base with pastry. Trim excess. Place in the fridge for 1 hour to rest.
Meanwhile, roll out half the reserved pastry to a 2mm-thick disc. Use 3cm, 4cm and 5cm flower-shaped cutters to cut blossoms. Place on a baking tray lined with baking paper. Divide remaining pastry into 2 portions. Tint 1 portion with green food colouring and remaining portion with yellow colouring. Roll out green pastry to a 2mm-thick disc. Use a teardrop-shaped cutter to cut leaves (alternatively, use a round cutter to cut out discs, then use a small sharp knife to shape into leaves). Place on the lined tray. Roll out yellow pastry to a 2mm-thick disc and use 5mm and 1cm round cutters to cut discs. Use a little water to attach discs to the top of the flowers. Place flowers and leaves in the fridge for 30 mins to rest.
Preheat oven to 200°C. Cover pastry with baking paper and fill with pastry weights or rice. Bake for 10 mins. Remove paper and pastry weights or rice. Bake for 8-10 mins or until light golden. Reduce oven to 180°C.
While the pastry is baking, cook the rhubarb, apple and sugar in a saucepan over medium heat, stirring occasionally, for 5-7 mins or until sugar dissolves and rhubarb is just tender. Combine cornflour with 2 tbs cold water. Add to the rhubarb mixture with strawberry. Cook, stirring, for 2 mins or until sauce thickens.
Spoon rhubarb mixture into pastry case and smooth the surface. Bake for 20 mins or until heated through.
Place flowers and leaves on a baking tray lined with baking paper. Brush with egg white. Sprinkle with sugar. Bake for 5 mins or until light golden. Set aside for 5 mins to cool. Arrange on tart.