Preheat oven to 350 degrees F. Butter and flour 2 (9-inch) round cake pans. Combine cake mix, nectar, eggs, and oil in large bowl. Beat with a hand mixer for 2 minutes, or until well blended. Divide batter between prepared pans. Bake for 25 minutes, or until a toothpick inserted into center comes out clean. Cool cakes in pans on cooling racks for 20 minutes. Invert cakes onto cooling rack and cool completely.
Thinly slice enough strawberries to yield 1 1/4 cups. Using a serrated knife, cut each cake horizontally in half, forming 4 layers in total. Place 1 cake layer, cut side down, on a serving platter. Spread 1/3 cup jam over top of cake layer. Arrange 1/4 cup of sliced strawberries in a single layer atop jam. Top with second cake layer, cut side down. Repeat layering with jam and sliced strawberries. Top with third cake layer, cut side down. Repeat layering with jam and sliced strawberries. Top with remaining cake layer, cut side down. Spread frosting evenly over top and sides of cake to coat completely. Arrange remaining 1/2 cup sliced strawberries and 1 whole strawberry decoratively atop cake.
Refrigerate cake for 40 minutes. Serve cake with remaining whole strawberries.