Toss chopped rhubarb with brown sugar, white sugar, orange zest, vanilla extract, cinnamon and flour.
Take a microwave safe mug and fill it with the rhubarb filling.
In a small bowl stir in the rolled oats, flour, brown sugar, white sugar and cinnamon. Cut the cold butter in with a pastry blender to create the crisp topping (or see top tip in post for grating frozen butter)
Spoon the crisp mixture over the top of the fruit filling. Microwave for about- 2 ½ minutes or until the butter is melted into the topping. Cooking time is based on my 1200W microwave so your timing might vary between 2-3 minutes.
Carefully remove from the microwave (I use an oven mitt) and let stand until cool enough to eat. Be careful because it's going to be really hot at first!
Enjoy warm or cold with whipped cream or a big scoop of vanilla ice cream and a sprinkle of cinnamon and additional orange zest.