Preheat your oven to 350°F (175°C). Rinse and slice rhubarb into 1-inch chunks. Toss with 1/4 cup of sugar and let sit for 10 minutes. Roast on a baking sheet for 8-10 minutes until tender, then cool.

In a mixing bowl, combine 1/2 cup wholemeal flour, 1/2 cup sugar, and 1 tsp cinnamon. Cut in 5 tbsp cold butter until crumbly. Refrigerate while preparing dough.

In a large mixing bowl, whisk 4 cups all-purpose flour, 3/4 cup sugar, 1 tsp salt, and 1 tbsp baking powder. Cut in 8 tbsp cold butter until crumbly.

Mix together 1 cup milk and 3/4 cup buttermilk. Add reserved rhubarb juices. Stir until combined, then pour into dry ingredients.

Fold wet into dry ingredients gently until combined. Cover and chill for 20 minutes.

On a floured surface, turn out dough to 1-inch thick. Add half the rhubarb on one side, fold, and roll gently. Cut into squares or triangles and place on a lined baking sheet.

Brush tops with buttermilk and sprinkle with crumble topping. Bake for 25-30 minutes until golden brown.