Preheat oven to 350 degrees. Combine flour, oats, brown sugar, cinnamon, and butter, blend well. Pat half of crumb mixture into a 9-inch baking pan (if placing in 9x13 dish increase ingredients by half, i.e. 6 cups rhubarb, 1 1/2 cups flour, etc); cover with rhubarb/berries.
Combine sugar and cornstarch; slowly add water, blending well. Cook over low heat, stirring occasionally, until smooth and slightly thickened. Stir in vanilla.
Pour sauce over rhubarb, and sprinkle with remaining crumbs. Bake at 350 degrees for 40 minutes or until rhubarb is tender. Serve warm with ice cream.
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