Add chopped rhubarb, 1 cup brown sugar, 3 tablespoons of flour, vanilla and salt to a bowl and stir to combine. Set aside.
Make Crumble Topping + Bake
Whisk together oats, remaining flour and brown sugar, cinnamon and baking powder. Pour melted margarine over and stir until well combined. It will be a little crumbly and that is fine.
Pour rhubarb mixture into a 9"x13" casserole dish or high sided baking pan. Sprinkle the crumble mix evenly all over and place in the oven to bake.Bake at 350'F for 40 minutes, until top is golden brown and rhubarb is tender, juicy and slightly thickened.
Serve warm with vegan vanilla ice cream or whipped cream.
Notes
This vegan rhubarb crumble recipe errs on the side of less sugar. If you prefer a sweeter rhubarb dessert, you can add an extra ¼ cup to ½ cup of brown sugar to the rhubarb mixture before arranging it in the baking dish.