In a medium saucepan over low heat, combine the cut plums, ½ cup light brown sugar, ground cinnamon, and star anise. Stir gently, releasing the juices, and cook for 5-7 minutes until the plums have softened but still hold their shape. Remove the star anise and set the mixture aside to cool.

Preheat your oven to 350°F (175°C). Line a 9x13 inch baking pan with parchment paper for easy removal. In a large bowl, mix the jumbo rolled oats, all-purpose flour, and 1 cup light brown sugar. Add the cubed butter to the dry ingredients and, using your fingers or a pastry cutter, mix until the mixture resembles coarse crumbs.

Pat half of the crumble mixture firmly into the bottom of the prepared pan. Spread the spiced plum mixture evenly over the crumble base. Top with the remaining crumble mixture, ensuring it’s evenly distributed. Bake for 30-35 minutes, or until the top is golden brown and the filling is bubbly.

Allow the bars to cool in the pan for at least 20 minutes, then lift them out using the parchment paper. Cut into squares and serve warm, perhaps with a dollop of whipped cream or a scoop of vanilla ice cream for extra indulgence.