For maximum flavor in these crispy-crusted bars, choose fresh, local blueberries that are at their peak ripeness.
1. In small bowl stir together cereal and flour. Add butter. Toss until combined.
2. Press cereal mixture onto bottom of 11 x 7 x 1 1/2-inch baking pan. Bake at 350°F for 6 to 9 minutes or until lightly browned. Cool completely in pan on wire rack.
3. Meanwhile, grate 1/2 teaspoon peel from lemon. Squeeze 1 tablespoon juice from lemon. Set aside.
4. In medium saucepan stir together lemon peel, cornstarch, sugar and salt. Gradually whisk in grape or apple juice and water. Stir in 1 cup of the blueberries.
5. Cook and stir over medium heat until mixture thickens and boils. Remove from heat. Stir in lemon juice. Let stand for 3 minutes, stirring occasionally.
6. Stir the remaining 2 1/2 cups blueberries into thickened mixture. Carefully spread over crust. Sprinkle with walnuts. Cover and refrigerate at least 1 hour or until set. Cut into eight bars. Store in refrigerator.