01 - Preheat the oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment, allowing a slight overhang for easy removal.
02 - Combine oats, flour, brown sugar, cinnamon, and salt in a large bowl. Stir in melted butter until the mixture resembles damp sand.
03 - Firmly press two-thirds of the dough into the prepared pan, creating an even base. Reserve remaining dough. Bake for 12 minutes or until lightly set.
04 - Toss plums with granulated sugar, lemon juice, and cornstarch in a medium bowl until well coated.
05 - Spread the plum mixture evenly over the baked crust.
06 - Mix reserved oat dough with walnuts, rolled oats, and brown sugar to create the crumble topping.
07 - Scatter walnut crumble topping evenly over plums. Bake for 35 minutes or until top is golden and fruit is bubbling.
08 - Allow bars to cool completely in the pan before lifting out and slicing into portions.