Preheat and prep: Heat your oven to 350°F (175°C). Lightly butter a 9-inch baking dish or similar.

Mix the berries: In a large bowl, combine strawberries, granulated sugar, 1/2 cup flour, lemon zest, lemon juice, vanilla, and cornstarch if using. Toss gently until the berries are coated. Pour into the baking dish.

Make the crumble: In another bowl, stir together oats, 3/4 cup flour, brown sugar, cinnamon, and salt. Add the cold butter. Use your fingers or a pastry cutter to work it in until clumps form. You want pea-sized pieces and some sandy bits.

Top it: Sprinkle the crumble evenly over the strawberry mixture. Don’t pack it down—loose crumbs crisp better.

Bake: Place the dish on a parchment-lined sheet pan to catch any drips. Bake for 35–45 minutes, until the topping is golden and the filling is bubbling around the edges.

Rest: Let it cool for at least 15–20 minutes. The juices will thicken as it sits.

Serve: Enjoy warm with ice cream, whipped cream, or a little plain yogurt.