Hull and roughly chop the fresh strawberries. Add to a medium saucepan with the sugar and lemon juice.

Cook over medium-low heat for 20–30 minutes, stirring occasionally, until the mixture thickens and slightly reduces.

In a small bowl, mix the cornstarch with 2 tablespoons of cold water to make a slurry. Stir into the strawberry mixture and continue to boil for 1 minute, stirring constantly, until thick and glossy.

Remove from the heat and let cool completely. The jam will thicken further as it cools. Refrigerate until ready to use.

Preheat the oven to 325°F. Grease and line three 8-inch round cake pans with parchment paper.

In a bowl, whisk together the flour, baking powder, baking soda, salt, and strawberry Jell-O powder. Set aside.

In a stand mixer fitted with the paddle attachment, or using an electric hand mixer, beat the softened butter, oil, and sugar together on medium speed for 2 minutes, until light and fluffy.

Add the eggs one at a time, mixing well after each addition.

In a separate bowl or liquid measuring cup, whisk together the milk, ½ cup of strawberry jam, and vanilla extract.

Add about one-third of the dry ingredients and mix until mostly combined, then add half of the wet ingredients and mix until just incorporated. Repeat with another third of the dry ingredients, then the remaining wet ingredients, finishing with the final third of the dry ingredients. Mix just until combined. If desired, add 1–2 drops of red food coloring for a deeper pink color. Scrape down the sides and bottom of the bowl, then give the batter a final gentle mix by hand to ensure everything is evenly incorporated.

Divide the batter evenly between pans (about 420g each). Bake for 20–25 minutes, or until it springs back to the touch, or a toothpick inserted in the center comes out with a few moist crumbs.

Cool in the pans for 10–15 minutes, then transfer to a wire rack to cool completely before assembling.

In a stand mixer fitted with the paddle attachment, or using an electric hand mixer, beat the softened butter on medium speed for about 2 minutes, until smooth and creamy.

Gradually add the softened cream cheese a little bit at a time, mixing well after each addition. Scrape down the sides and bottom of the bowl and mix until smooth and lump-free.

Add the powdered sugar and salt. Mix on low speed until incorporated, then increase the speed and beat until light and fluffy.

In a small bowl, strain ¼ cup of the strawberry jam so that only the liquid remains and no strawberry pieces. Whisk in 1 teaspoon of the strawberry Jell-O (optional) and the vanilla extract. Add the mixture to the frosting and mix on low speed until combined.

Cover and refrigerate until the cakes are fully cooled and ready to assemble.

If the cakes are slightly domed, trim the tops to level each layer.

Place the first cake layer on a serving plate or cake stand. Spread about 1 cup of frosting over the top. Pipe a border around the edge, or use an offset spatula to create a dam of frosting around the edge, then fill the center with about ½ cup of strawberry jam.

Add the second cake layer and repeat with the frosting and jam filling. Place the final cake layer on top.

Spread a thin layer of frosting over the entire cake to create a crumb coat. Chill for about 15 minutes to set so the cake doesn’t shift when adding the final coat of frosting.

Frost the cake with the remaining frosting, smoothing the sides and top.

Decorate as desired. Fill a piping bag with the remaining frosting and pipe a border on top of the cake. Spoon the strawberry jam inside the border, and decorate with fresh strawberry slices around the sides and top.