Grease tart tin with butter, line with pastry and rest in freezer foe 30mins.
Preheat oven 180'. Place baking paper over tart shell and fill with baking weights. Blind bake for 15 minutes.
Allow to cool while making filling.
In a processor - cream butter and sugar together until light and creamy. Add egg, zest and cream and mix well.
Add almond and flour, mix slowly.
Arrange cut rhubarb on the bottom of tart shell. Dollop spoonfuls of frangipane mix over rhubarb and spread around until covered.
Bake for 30 minutes or until golden and just set.
To serve - dust with icing sugar and crumble with mandarins.