Preheat Oven & Prep Pan:
Preheat oven to 325°F (163°C). Line a 13×9-inch baking pan with parchment paper and lightly grease with cooking spray.

Make the Cake Batter:
In a large bowl, whisk together the cake mix and pudding mix. Add lemonade, oil, and eggs, whisking until smooth. Pour the batter into the prepared pan and spread evenly.

Bake the Cake:
Bake for 45 minutes or until a toothpick inserted comes out clean. Remove from oven and let cool completely on a wire rack (about 30 minutes).

Prepare the Strawberry Filling:
In a small saucepan, bring lemonade to a low simmer over medium heat. Remove from heat, stir in strawberry Jell-O, and mix until fully dissolved (about 3 minutes). Set aside to cool to room temperature (about 10 minutes).

Poke & Fill the Cake:
Once the cake is cooled, use the handle of a wooden spoon to poke about 24 holes evenly across the cake. Spoon the Jell-O mixture into the holes.

Chill the Cake:
Cover and refrigerate for at least 6 hours (preferably overnight for the best texture).

Frost & Garnish:
Remove the cake from the refrigerator. Spread the buttercream frosting evenly over the top. Garnish with fresh strawberries and lemon slices before serving.