Preheat the oven to 180 degrees C. Grease and flour a 20cm square baking dish.
Place the rhubarb into the baking dish and sprinkle 3/4 cup of sugar over it. Stir to coat completely. In a medium bowl, stir together the flour, baking powder, salt and remaining 3/4 cup of sugar. Add the walnuts, egg and sour cream; stir until smooth. Pour over the rhubarb and spread evenly.
Bake in the preheated oven until rhubarb is tender and the top is browned, about 40 minutes.