Heat oven to 375 degrees. Lightly grease 13 x 9" pan. Combine first 5 ingredients. Add butter; stir with fork until mixture resembles coarse crumbs. Press half of crumb mixture into bottom of pan. Spread rhubarb sauce evenly onto crumb layer. Top with remaining crumbs and chopped walnuts. Bake 25 to 30 minutes or until top is golden brown and rhubarb is bubbly. Cool to room temperature before cutting.