Before you start mixing your dough, get your baking environment ready. Preheat your oven to 350°F so it is hot and steady by the time your bars are ready to go in.
In a large mixing bowl, combine your all-purpose flour, rolled oats, brown sugar, baking soda, and salt. Use a fork or a pastry cutter to stir these dry ingredients until the brown sugar lumps are broken down and everything is uniform.
Pour in your melted butter and vanilla extract. Stir the mixture until it looks like wet sand and starts to form small clumps.
Take about two-thirds of your oat mixture and dump it into the bottom of your prepared baking pan. Use the bottom of a flat measuring cup or your clean fingers to press the crumbs down firmly.
Wash your fresh strawberries and remove the green tops. Slice the berries into small, even pieces about the size of a blueberry. Place the diced fruit into a medium bowl and toss them with the granulated sugar, cornstarch, and lemon juice.
Spread the strawberry mixture evenly over the top of your pressed oat base. Use a spoon to push the fruit pieces around until the entire surface is covered. If you have a few extra berries, feel free to toss them in, but try not to make the fruit layer so thick that it prevents the top crumble from sticking.
Take the remaining third of your oat crumble and sprinkle it over the top of the strawberries. Do not press this layer down; let it stay loose and chunky. Slide your pan into the center rack of the oven. Bake for thirty to thirty-five minutes.
You must let the pan cool completely on a wire rack for at least one to two hours. If you try to cut the bars while they are still warm, the strawberry layer will be too liquid, and the bars will fall apart.
