In a bowl combine flour, packed brown sugar, quick-cooking oats, cinnamon, and nutmeg. Cut in the butter until the mixture is crumbly. Press half of the mixture into an ungreased 9 inch round baking pan.
Combine chopped rhubarb and strawberries; spoon into baking pan.
In a saucepan combine sugar, cornstarch, water, and vanilla. Bring to a boil over medium heat. Cook and stir for 2 minutes. Pour mixture over fruit. Sprinkle fruit with the remaining crumb mixture.
Bake at 350 degrees F (175 degrees C) for 1 hour. Wonderful when served with vanilla ice cream.