Preheat oven 325ºF and line an 8X8 baking dish with parchment paper.
In a small sauce pan combine rhubarb, brown sugar and water. Bring to a boil then allow to simmer for 8 to 10 minutes or until rhubarb has all broken down. Add the lemon and vanilla and stir. Allow to cool for 5 minutes before using.
Mix flour, oats, brown sugar, lemon zest, salt, cinnamon and baking soda in a bowl. Add butter and combine with hands. Mixture will be crumbly.
Reserve ¾cup of the flour/oat mixture and press the rest evenly into the bottom of the prepared baking dish. Spread rhubarb mixture evenly on top of that and sprinkle remaining flour/oat mixture over the rhubarb.
Bake on middle rack for 40 minutes. Allow to cool completely in the pan on a wire rack. Store in airtight container in the fridge for up to a week. Enjoy!
Recipe by pretty plain janes at https://prettyplainjanes.com/2016/05/18/oatmeal-rhubarb-crumb-bars/