In a medium sauce pan combine rhubarb, strawberries and lemon juice (do not add the sugar or cornstarch yet) cover and cook on medium heat, stirring occasionally. Cook for about 8-12 minutes until the fruit is tender.

In a small bowl combine the sugar and cornstarch together then gradually add it into the fruit mixture, stirring constantly. Mixture will come to a boil. Continue to stir for about 1 minute until mixture is thickened. Remove from heat and set aside to cool.

Preheat oven to 400 Degrees F. Grease a 13x9x2 inch pan and set aside.

In a medium mixing bowl sift together the flour, salt and baking soda. Set aside. In a large bowl cream together the butter and sugar. Add the dry ingredients into the the butter mixture and mix to moisten. Add nuts if you are using them.

Press 3-4 cups of the crumb mixture in to the bottom and half way up the side of a greased 13x9x2 inch pan. Place in oven and bake for 10-12 minutes.

Spread filling over partially baked crust. Sprinkle with remaining crumb mixture. Turn oven down to 350 Degrees F and bake for 25-30 minutes. Set aside to cool.