Hull and slice the strawberries into 1/4-inch thick pieces, then transfer to a colander and let them drain for a few minutes to remove excess liquid—this prevents a watery filling. While the berries drain, whisk together the cornstarch and water in a small bowl until completely smooth with no lumps, which ensures the thickener will incorporate evenly into the hot filling without creating clumps.

Combine the drained strawberries, sugar, lemon juice, and salt in a medium saucepan over medium-low heat. Stir occasionally and let the mixture heat gently until it reaches a simmer—this allows the sugar to dissolve while the berries release their natural juices and develop deeper flavor. Once simmering, reduce heat to low and cook for 6-8 minutes, stirring occasionally, until the strawberries are soft and the mixture has developed a glossy syrup around them.

Pour the cornstarch mixture from Step 1 into the simmering strawberry filling while stirring constantly to incorporate it evenly and prevent lumps. Continue cooking on low heat for 2 minutes, stirring frequently, until the mixture thickens noticeably and becomes glossy—you'll see it coat the back of a spoon. Remove from heat and let it cool to room temperature, about 15-20 minutes, then transfer to a container and refrigerate for at least 12 hours. I always make this filling a day ahead because the flavors mellow and intensify beautifully while chilling, and the texture sets up perfectly for layering into a cake.