Mix the flour, salt and icing sugar in a bowl. Add the margarine and rub together using your hands to form an even, crumbly mixture. Pour in the water and mix quickly to form a soft dough; do not knead. Shape the dough into a ball, wrap in cling film, place in the fridge for approx. 1 hr.
Mix all the ingredients in a small pan, bring to the boil, reduce the heat, simmer for 10-12 mins. until the rhubarb is soft but not mushy. Leave the filling to cool.
To shape & bake
On a lightly floured surface, roll the dough out into a rectangle, cut out 4 circles (e.g. using a muesli bowl). Spread the filling in the middle of the circle, fold up the edges. Transfer the galettes to a baking tray lined with baking paper. Melt the coconut oil, mix with the sugar, brush the galettes with the glaze.
Bake for 35-40 mins. in the centre of an oven preheated to 180°C.