Heat oven to 375 degrees F. Coat 15 x 10-inch jelly-roll pan with nonstick vegetable-oil cooking spray. Line bottom of pan with waxed paper; lightly coat waxed paper with cooking spray.

Cake: Beat egg whites and cream of tartar in large bowl until foamy. Gradually add 1/4 cup granulated sugar and beat until soft peaks form.

Beat together oil and remaining granulated sugar in second large bowl until well mixed, about 1 minute. Beat in vanilla.

Stir cake flour into oil mixture. Gently fold in a quarter of the egg whites until completely blended, then gently fold in remaining egg whites. Spread the batter evenly with a spatula in the prepared pan.

Bake in 375 degrees F oven for about 12 minutes or until top is very lightly browned and springs back when lightly touched.

Run knife around edge of cake to loosen. Sprinkle top with 3 tablespoons confectioners' sugar. Cover with clean kitchen towel. Top with slightly larger baking sheet; invert. Remove pan, then waxed paper. Starting at shorter end, roll up cake in towel. Cool completely, seam side down, on wire rack.

Filling: Stir together cream cheese, sour cream and strawberry jam in medium-size bowl. Gently fold in strawberries.

Assemble: Unroll cake. Spread cream cheese mixture over cake, leaving 1/2-inch border all around edges of cake. Carefully re-roll cake. Refrigerate for at least 1 hour.

To serve: Place cake, seam side down, on serving platter. Trim ends of cake. To garnish, sprinkle roll evenly with confectioners' sugar. Fan thin strawberry slices down center of cake, if desired.