To prepare the filling: toss together the rhubarb, sugar, flour and cinnamon in a large bowl until all the rhubarb is thoroughly coated. Transfer into an 8x8" baking dish.
4 cups (500g) rhubarb, 3/4 cup (150g) granulated sugar, 1/4 cup (30g) all-purpose flour, 1 tsp cinnamon
To prepare the topping: mix together the flour, rolled oats and brown sugar. Pour in the melted butter and mix until all is moistened. Using your hands, break up the streusel into bite-size pieces and sprinkle evenly over top of the filling.
1 cup (125g) all-purpose flour, 1/2 cup (45g) rolled oats, 1/2 cup (100g) packed brown sugar, 1/2 cup (114g) unsalted butter, melted
Bake for 40-45 minutes or until the top is lightly browned and the filling is bubbling.
Serve warm or cold with some sweetened whipped cream or vanilla ice cream if desired.
Notes
Cover and store in the fridge for 3-4 days. Please note that the streusel will get soggy over time, so it’s best consumed within a day or two.