Preheat oven to 160oC fan-forced. Grease a 6 cup capacity oven proof dish.
Combine rhubarb, apple, caster sugar and 2 tablespoons of cold water in prepared dish. Cover with foil. Bake for 20 to 25 minutes or until fruit is just tender.
Meanwhile, sift flours and mixed spice into a bowl. Using fingertips, rub butter into flour mixture until mixture resembles breadcrumbs. Stir in oats, brown sugar, coconut and almonds.
Remove and discard foil from fruit. Sprinkle with crumble mixture. Bake for a further 20 to 25 minutes or until crumble is golden. Serve with custard.