Preheat your oven to 350°F. While it heats, grease a 9x13 inch baking dish thoroughly with butter or cooking spray, making sure to coat the bottom and sides evenly. This prevents the bars from sticking and ensures clean edges when you cut them.

In a large bowl, combine the flour, rolled oats, brown sugar, baking powder, and salt. Whisk together until evenly distributed. Pour in the melted butter and vanilla extract, then stir with a wooden spoon or spatula until the mixture resembles coarse breadcrumbs with small clumps throughout. I like to use my hands at the end to gently work the butter through—it helps you feel when you've reached the right texture without overworking the dough.

Press half of the oat mixture firmly and evenly into the prepared baking dish, creating an even bottom layer. Spread the strawberry preserves in a thin, even layer over the oat base, leaving about a quarter-inch from the edges. Stir the lemon juice into the preserves right before spreading to add brightness and balance the sweetness—it's a subtle touch that makes these bars taste less one-dimensional.

Sprinkle the remaining oat mixture evenly over the jam layer, distributing it to the edges. Gently press down with the back of a spoon or your hands so the topping adheres slightly to the jam without compacting too much—you want it to stay crumbly and textured. Bake for 35 to 40 minutes, until the top is light golden brown and the edges are slightly darker. The bars will continue to firm up as they cool, so don't overbake.

Remove the baking dish from the oven and place it on a cooling rack or wire rack. Let the bars cool completely at room temperature—this takes about 1 to 2 hours. Once cooled, run a thin knife around the edges to loosen them from the pan, then cut into squares of your desired size. The bars will be delicate while warm, so patience here ensures clean cuts and a professional appearance.