Step 1

Preheat oven to 350F. Lightly butter an 8x8" square pan and line with a piece of parchment paper, leaving an overhang on either side. Lightly butter the parchment. Set aside.

Step 2

Prepare the filling. In a small saucepan set over medium heat, bring the filling ingredients to a simmer. Cook, stirring occasionally, for about 10 minutes or until the filling is thickened and rhubarb is starting to break down. Remove from heat and set aside to cool.

Step 3

Meanwhile, prepare the crumble crust. In a large mixing bowl, combine flour, oats, sugar, salt, baking powder and cinnamon. Drizzle in the melted butter, and stir until the mixture forms loose crumbs.

Step 4

Set aside 1 cup of the mixture for the topping. Press the remainder into the prepared baking pan, using the bottom of a glass or measuring cup to form an even layer. Spread the cooled filling onto the crust. Crumble the reserved topping evenly onto the filling, then scatter chopped walnuts over top.

Step 5

Bake in preheated oven for 30-35 minutes, or until the filling is bubbly around the edges and topping is lightly golden. Let cool completely in the pan, then transfer to a cutting board and cut into 12 bars. Bars will keep for up to 3 days in an airtight container (assuming they actually last that long).