Preheat the oven to 180C / 350F and grease a 3x9-inch pan with oil.
Make the cake: In a mixing bowl, mix together the cake mix, 180 ml of water, sunflower oil and the eggs until combined.
Transfer the batter to the baking pan and bake for 20-25 minutes.
Prepare the jello: In a saucepan, add 350 ml water and bring to a boil. Then, turn off the heat, add the strawberry jello, and stir until it dissolves.
While that cake is still warm poke holes evenly all over the cake with a skewer (I use a chopstick) and carefully spoon the hot strawberry jello over the cake, letting it soak in thoroughly. Put the cake in the fridge for at least 4 hours to let the Jello set.
Prepare the topping: In a stand mixer bowl, add the cold heavy cream and powdered sugar and whip until a stiff peak forms.
Take the cake out of the fridge and spread the cream on top, decorate with strawberry slices and enjoy.
Notes
Keep leftovers in a well-sealed container in the fridge; they’ll stay fresh for 2-3 days!