Combine rhubarb, sorrel or strawberries with sugar orange peel and vanilla in large saucepan. Bring to a boil over medium-high heat, then reduce to medium and cook 4 minutes, stirring frequently.
Disolve cornstarch in water. Add to rhubarb mixture and cook until thickened, stirring constantly. Set aside
Mix flour, oats, brown sugar, butter, salt, cinnamon, and nuts together until crumbly. Place about 3 1/2 cups of crumb mixture into greased 9x13 inch baking pan and press to make an even layer. Pour in rhubarb/sorrel mixture and spread evenly. Sprinkle remaining crumb mixture over top. Bake in preheated oven at 350 degrees for 30-40 minutes. Cut into squares.