Preheat the oven to 180C/350F/Gas 4.
For the crumble filling, melt the butter in a small saucepan, add the rhubarb, apple and ginger and cook for three minutes, or until softened. Stir in the cornflour and cook for one minute. Spoon into a small ovenproof dish.
For the topping, place the butter, sugars and flour into a blender and process until the mixture resembles breadcrumbs. Stir in the porridge oats and then scatter over the fruit in the dish. Place on a baking tray and bake in the oven for ten minutes, or until the top is golden-brown.
For the custard, place the double cream, coconut milk and vanilla extract into a saucepan and bring almost to the boil.
In a bowl, whisk together the egg yolks and sugar until pale and frothy. Whisking continuously, pour the hot cream mix onto the egg mix and beat well. Pour back into the saucepan and stir until thickened slightly.
Serve the coconut custard with the crumble.