Preheat oven to 375F. Combine first four ingredients in a bowl. Set aside. Combine rhubarb, 1/4 cup flour, and 1/2 cup sugar in a bowl and stir well. Set aside until sugar dissolves and mixture looks wet.
Brown the butter. Cut in small chunks and cook over medium heat stirring frequently. Butter is done when it smells toasty and appears darkened. Remove from heat and mix with dry ingredients. Reserve 1/2 cup of crumble for topping the bars.
Line 7×11″ or 8x8' baking pan with parchment paper. Press crumble mixture into pan, top with rhubarb, and sprinkle with reserved 1/2 cup of crumble. Bake for 40 – 50 minutes. Let cool completely before cutting.
Variations. Replace butter with melted coconut oil for vegan version. Sub gluten free (Pamela’s Baking and Pancake Mix) or spelt flour.
(Crumble & Crust adapted from One Bowl Baking.)