Preheat the oven to 375 degree. Lightly oil a 9 x 9 baking dish. Set aside.
In a large bowl, place the chopped rhubarb, sliced strawberries, lemon juice, vanilla, brown sugar, cinnamon, and arrowroot. Stir gently until evenly distributed. Place mixture and spread evenly in prepared baking dish. Set aside.
In a medium sized bowl, combine the dry ingredients for the crisp topping--brown sugar, salt, oats, chopped walnuts, and cinnamon. Mix until evenly distributed.
Add the margarine to the topping ingredients. Using your fingers or a fork, mix in the margarine until it is broken up.
Using your fingers, evenly distribute the topping over the fruit mixture.
Bake at 375 degrees for 30 minutes or until fruit is bubbling, and topping is lightly browned.
Let the dessert sit for about 15 minutes before serving. Enjoy.