Place the coconut milk in the fridge overnight, skim off the hard upper layer, set aside for the filling (the rest can be used for smoothies, etc.).
Mix the flour, sugar and salt in a bowl. Add the margarine and rub together by hand to form an even, crumbly mixture. Pour in the water and mix quickly to form a soft dough; do not knead. Flatten the dough, cover and chill for approx. 30 mins. Roll out the dough on a lightly floured surface, transfer to the prepared tin, firmly prick the base with a fork. Cover the dough with baking paper and weigh down with dried pulses.
Blind bake for approx. 15 mins. in the lower half of an oven preheated to 220°C. Remove the baking paper and dried pulses, leave the pastry to cool in the tin.
Combine the lemon juice, zest, sugar and cornflour in a pan. Bring to the boil while stirring with a whisk, reduce the heat. Stir in the reserved coconut layer, return to the boil, stir and simmer for approx. 1 min. until the mixture thickens. Allow to cool slightly, pour into the tart base.
Bake for approx. 15 mins. in the lower half of an oven preheated to 180°C. Leave the tart to cool.
Scatter the passion fruit seeds on top of the tart.