Place plain flour, rolled oats, and cold cubed butter into a large mixing bowl. Rub the butter into the flour using your finger tips until well mixed and like breadcrumbs. Set to one side
In a separate large mixing bowl mix together the sliced plums, chopped rhubarb, 150g sugar, and spices. Stir until the sugar and spice fully coats the fruit.
Transfer the fruit mixture to an ovenproof dish or cast iron pan. Spread the fruits out to cover the base of the dish. Scatter the crumble mixture over the top of the fruit. Then sprinkle the remaining 50g of sugar evenly over the top of the crumble.
Place the plum and rhubarb crumble in the center of the oven and bake for 30-40 minutes or until bubbling and golden brown.