Cut the rhubarb and add it to a pan with the juice and orange peel. Add the vanilla and sugar and heat over a high heat for 2–3 minutes.
Chop the strawberries and add them to a large baking dish along with the blueberries and cornmeal. Add the rhubarb and juices and mix gently.
For the cobbler, put the flour in a bowl with the butter and rub with your fingers until it resembles coarse breadcrumbs. Add the sugar and oats and mix. Pour in the yoghurt and bring the mixture together using a knife.
Roll pieces of the cobbler mixture into balls of different sizes and place them on top of the fruit. Sprinkle with Demerara sugar and oats and bake for 35–40 minutes until the juices are bubbling and the cobbler topping is golden brown. Serve with cream or ice cream.