Prepare a 13 x 9 inch baking dish with cooking spray.
In a medium size mixing bowl, cream together brown sugar, 1/4 cup of butter, oil and egg.
In a separate bowl, sift together flour, soda and salt.
Add dry ingredients to wet ingredients, alternating with yogurt and stirring between each.
Stir in rhubarb.
Pour mixture into baking dish.
In a small mixing bowl, combine sugar, walnuts, 1 tbsp of butter and cinnamon until crumbly.
Spread crumb mixture evenly across the baking dish.
Bake at 350°for 45 to 50 minutes.
Remove from oven and let cool.
Cut in squares and serve warm or cool.
Try it with a scoop of Stonyfield Organic Gotta Have Vanilla Frozen Yogurt or Ice Cream.