Make the plum caramel. Blend together 6 ounces chopped plums with the lemon juice until smooth.

Melt ½ cup sugar in a medium sauce pot over medium-low heat until amber brown, about 5 minutes. Remove from heat and stir in the pureed plum mixture. Return the pot back to heat and continue to simmer for another 5 minutes or until thickened, stirring frequently.

Preheat oven to 375 degrees F. Line a 9x13 inch baking pan with parchment paper and spray with cooking spray.

Make the crumble. Combine the flour with rolled oats, brown sugar, butter, egg, and baking powder, and salt in the bowl of a stand mixer. Beat the crumble together until the butter is crumbly.

Toss together the plums with the plum caramel and cornstarch. Spread ⅔ of the crumble mixture into the bottom of the prepared pan, pressing down to create one even crust. Bake the crumble for 10 minutes.

Remove the crumble from the oven and spread the plums in an even layer along with all of the juices. Sprinkle the remaining crumble on top and bake the dessert for 25 minutes or until golden brown on top. Remove from heat and let cool completely. Cut into bars and serve.