Step 1

Preheat the oven to 350 degrees F (175 degrees C). Coat an 8x4-inch loaf pan with nonstick cooking spray.

Step 2

Mix water and sugar in a deep saucepan, making sure sugar is completely mixed in. Bring to a low boil over medium heat. Stir in rhubarb and let simmer, uncovered, until rhubarb becomes mushy, about 10 minutes. Add a couple tablespoons water if needed to help soften. Remove from heat and set rhubarb aside in a bowl.

Step 3

Stir oats, whole wheat flour, and cinnamon together in a bowl. Make a well in the center of the oat mixture. Pour in maple syrup and butter; stir until all ingredients are incorporated. Set topping aside.

Step 4

Whisk all-purpose flour, baking powder, and salt for crust together in a small bowl.

Step 5

Whisk cashew milk, butter, and vanilla extract together in a bowl. Stir in sugar. Pour into crust flour mixture, stirring just until incorporated.

Step 6

Press crust into the bottom of the prepared pan using a spatula; wet your spatula with water to prevent sticking. Spread rhubarb filling evenly over the crust. Sprinkle oat topping mixture on top of rhubarb.

Step 7

Bake in the preheated oven until juices are bubbling and crust is golden, 15 to 19 minutes. Let cool completely to room temperature in the pan. Let bars rest for at least 3 hours before slicing and serving.