Rhubarb and Strawberry Crisp

from Simply in Season

4 cups rhubarb (finely chopped)
2 cups sorrel (finely chopped) or strawberries (sliced)
1 cup sugar
1 tablespoon orange peel (grated)
1 teaspoon vanilla
Combine in a large saucepan. Bring to a boil over medium-high heat, then reduce to medium and cook 4 minutes, stirring frequently.

1/4 cup water
3 tablespoons cornstarch
Dissolve cornstarch in water. Add to rhubarb mixture and cook until thickened, stirring constantly. Set aside.

1 1/2 cups flour (may substitute part oat bran)
1 1/2 cups rolled oats
3/4 cup brown sugar
3/4 cup butter
1/2 teaspoon salt
1/2 teaspoon ground cinnamon
1/2 cup walnuts or other nuts (chopped; optional)
Mix together until crumbly. Place about 3 1/2 cups of crumb mixture into greased 9 x 13-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 350F for 30-40 minutes. Cut into squares.