In a medium saucepan over medium heat, combine the rhubarb, 2 tablespoons of the water, strawberries and sugar and simmer for approximately 10 minutes, until the rhubarb gets soft and starts to break down, stirring occasionally and adding a drizzle more water if needed to prevent it from scorching. 

Combine the corn starch with the remaining 2 tablespoons of water and stir to dissolve. Pour the cornstarch mixture into the bubbling fruit mixture and simmer for 1-2 minutes, until it is thickened and no longer cloudy. Remove from the heat and set aside to cool.

Preheat the oven to 350 F and line a lightly greased 9x13" baking pan with parchment paper leaving extra to over hang the sides (I use a cooking spray for this step, it helps the parchment stick to the pan).

Combine all of the oat crumble ingredients together in a large bowl and stir well to combine. Press 2/3 of the oat mixture evenly into the bottom of the pan, and spread the cooled fruit mixture using a rubber spatula on top. Sprinkle the remaining 1/3 oat mixture evenly on top of the bars and bake for approximately 35-40 minutes, until the bars are golden brown. Remove from the oven and cool before serving.

Once cooled, remove the bars from the pan (using the overhanging parchment to pull them out) and cut the bars with a good quality sharp knife. Serve on their own or with a small scoop of ice cream. Store the bars in the fridge, covered, for up to 5 days or frozen for up to 1 month.