Preheat the oven according to the directions on 1 box lemon cake mix.
Prepare the cake batter according to the package directions, using the eggs, vegetable oil, and water called for on the cake mix box.
Pour the batter into the prepared baking pan and bake according to the package directions.
Remove the cake from the oven and let it cool for about 5 minutes.
In a medium bowl, whisk together 1 package (3 ounces) lemon gelatin and 1 cup hot water until the gelatin is completely dissolved.
Whisk in 1 cup cold water.
Slowly pour the gelatin mixture evenly over the cake, allowing it to soak into the holes.
Refrigerate the cake for 2 to 3 hours until completely chilled.
In a large mixing bowl, beat 1 tub lemon frosting with a hand mixer until light and fluffy.
Fold in 1/2 tub (4 ounces) whipped topping until fully combined.
Spread the frosting evenly over the chilled cake.
Refrigerate the cake for about 2 hours before serving.
If desired, garnish with fresh lemon slices before serving.