PREHEAT oven to 350°F. Beat egg, sour cream, cream cheese and cake mix in large bowl with electric mixer on low speed just until cake mix is moistened, scraping side of bowl often. Blend in 1/4 cup of the butter. Beat on medium speed 2 min. (batter will be thick).
SPREAD batter into lightly greased 13x9-inch baking pan. Arrange pear slices evenly on batter, pressing lightly into batter. Sprinkle evenly with dry jelly powder mix and cinnamon. Drizzle evenly with remaining 3 Tbsp. butter; sprinkle with walnuts.
BAKE 40 to 45 min. or until toothpick inserted in centre comes out clean. Serve warm or at room temperature. Cut into 24 pieces. Cover and refrigerate leftover cake.