Place rhubarb in saucepan with ¼ cup water and 1 tsp sugar. Heat until rhubarb softens. Scatter sliced apple in an oven proof dish and place stewed rhubarb on top.
Place all custard ingredients into a saucepan and cook until sauce thickens and resembles custard. Pour custard over the apple and rhubarb.
Place all crumble ingredients into a food processor and mix until mixture resembles crumbs.
Sprinkle crumble over custard and bake for 30 mins. Serve immediately with thick cream or ice cream!