Put flour, butter in flakes, 50 g sugar and 1 tablespoon cold water in a bowl. Knead first with the dough hooks of the hand mixer, then with your hands to a smooth shortcrust pastry. Wrap the dough in foil and chill for about 30 minutes.
Wash, clean and cut the rhubarb into pieces. Bring rhubarb, apple juice, 50 g sugar and grenadine syrup to the boil and simmer for about 10 minutes. Mix the pudding powder with some water, stir into the rhubarb, bring to the boil again.
Take it off the stove.
Roll out the short pastry rectangularly (approx. 13 x 37 cm) on a floured work surface. Using a dough roller, place the dough in a greased, flour-sprinkled rectangular tart tin (11 x 35 cm; with lift-off base).
Press the edges and remove any protruding edge of the dough. Prick the tart several times with a fork and pre-bake in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for approx. 10 minutes.
Spread the compote on the pre-baked short pastry and bake for approx. 40 minutes. In the meantime, beat the egg white and salt with the whisk of the hand mixer until stiff, adding 100 g sugar.
Pour the beaten egg whites into a piping bag with a star-shaped spout and spread in tuffs on the rhubarb. Continue baking at the same temperature for about 10 minutes. Remove and let cool on a cake rack.