Prepare the pan: Preheat your oven to 325°F (163°C). Line a 9x13-inch baking dish with parchment paper and leave enough overhang on both sides to use as handles when lifting the bars out later. Lightly grease the parchment.

Make the strawberry swirl: In a medium saucepan, combine the frozen strawberries, sugar, and lemon juice. Cook over medium heat, stirring occasionally, until the berries break down and release their juice — about 8–10 minutes. Stir in the cornstarch slurry and cook another 1–2 minutes until the sauce thickens noticeably. At this point you can blend it slightly for a smoother sauce or leave it a bit chunky for more texture. Transfer to a bowl and let it cool completely before using. This step can be done an hour ahead or even the day before.

Make the crust: In a large bowl, mix the graham cracker crumbs, melted butter, and sugar until the mixture looks like wet sand and holds together when squeezed. Pour it into the prepared pan and press it into an even layer using your hands first, then compact it firmly with the flat bottom of a measuring cup or drinking glass. Get the edges tight. Bake for 10 minutes, then set aside to cool slightly while you make the filling.

Make the cheesecake filling: Beat the room temperature cream cheese with an electric mixer on medium speed until completely smooth, about 2 minutes. Scrape the bowl down. Add the sugar and beat until fully incorporated. Add the eggs one at a time, mixing on low speed after each one — low speed is important here because you don't want to whip air into the batter, which can cause cracking. Mix in the vanilla and salt. Stop as soon as everything is combined.

Pour the cheesecake filling evenly over the slightly cooled crust and spread it to the edges with a spatula. Drop spoonfuls of the cooled strawberry sauce across the top — aim for about 20 small dollops spread evenly rather than a few big ones. Use a butter knife or skewer to gently swirl: drag it through in slow S-curves without going all the way to the bottom. You want defined ribbons of strawberry, not a pink filling. Less swirling gives you better visual definition.

Make and add the crumble. In a bowl, whisk together the flour, brown sugar, and salt. Pour in the melted butter and stir until the mixture forms clumpy, uneven crumbles — some larger, some smaller. Don't overwork it or it'll turn into a paste. Sprinkle the crumble evenly over the top of the assembled bars. Don't press it in.

Bake uncovered at 325°F for 40–45 minutes. The center should be just set and have a slight jiggle when you gently shake the pan — it will firm up fully as it chills. The crumble on top should be golden. If the crumble is browning too fast before the center sets, loosely tent with foil.

Cool the bars completely at room temperature, then refrigerate for at least 4 hours — overnight is better. Use the parchment overhang to lift the whole slab out onto a cutting board. Slice into bars with a sharp knife, wiping the blade clean between cuts for the cleanest edges.