Preheat oven to 375 degree F. In a medium bowl combine the 2/3 cup granulated sugar, the cornstarch, and cinnamon. (For fresh rhubarb, use 2 teaspoons cornstarch. For frozen, use 3 teaspoons cornstarch.) Stir in rhubarb, strawberries, and basil. Spoon into bottom of a 2-quart square baking dish; set aside.
In another medium bowl combine flour, oats, brown sugar, and salt. Stir in melted margarine or butter. Sprinkle over fruit. Bake 30 to 35 minutes or until fruit is tender and topping is golden brown. Serve warm. Makes 6 servings.