1Heat oven to 350°F. In 13x9-inch pan, melt butter in oven. Sprinkle brown sugar and walnuts over butter; stir to combine. Sprinkle chopped apples in pan.
2Mix cake as directed on box for 13x9-inch pan, using water, oil and eggs. Pour batter over apples and walnuts in pan.
3Bake 35 to 40 minutes or until toothpick inserted in center comes out clean. Cool on cooling rack 2 minutes; run knife around sides of pan to loosen cake. Place heatproof serving plate upside down on pan; turn plate and pan over. Remove pan. Serve cake warm.