Preheat oven to 350°F. Line a 9 x 9 pan with foil, grease the foil. Set aside.: There is a warm, anticipatory smell when the oven comes to temperature, and lining the pan with foil makes it simple to lift the bars out later. Greasing the foil ensures the base releases cleanly, preventing any tearing when you slice. A common misstep is skipping the foil and struggling to remove the bars, which can ruin their neat edges, so take the extra minute here. You should feel the oven hum and see a steady orange glow when it is ready, which signals even heat for a consistent bake.
With a hand mixer or stand mixer, beat butter and brown sugar for several minutes, until creamy.: As you cream the unsalted butter and light brown sugar , you will notice the mixture lighten in color and increase in volume, becoming silky and glossy. This texture traps tiny air pockets that contribute to a tender crust, and the molasses in the brown sugar gives a warm, caramel aroma. If the butter is too cold, it will not cream properly, leaving clumps and a grainy finish, so allow it to soften. The sound shifts from a soft whirl to a smoother hum as the mixture emulsifies, and you will see a ribbon fall from the beaters when it is ready.
In a separate bowl, combine almonds, flour, oats, baking soda and salt. Add the dry ingredients to the butter mixture and beat on low speed until blended and crumbly. Remove 1 1/4 cups of the mixture and set it aside.: The dry mix itself smells nutty and wholesome, the sliced almonds offering a toasted scent even before baking. When you fold it into the creamed mixture, the texture changes to a coarse, sandy crumb which is exactly what you want for both the crust and the streusel. Taking out the reserved portion ensures you have enough for a crunchy topping, and it prevents overpacking the pan. A frequent error is overworking the mix at this stage, which can develop the gluten in the white whole wheat flour and result in toughness, so mix only until combined.
Dump the remaining mixture into the prepared 9 x 9 pan. Use the bottom of a measuring cup to evenly press the mixture into the pan. Spread the strawberry jam over the crust, then top with sliced strawberries. Sprinkle the reserved oat mixture over the top.: Pressing the base with a flat bottom creates an even, compact crust that bakes into a stable platform for the jam and fruit. You will feel a tactile resistance as the crumbs compress, and the surface should look uniformly packed without gaps. Spreading a thin layer of strawberry jam ensures it warms and bubbles without sliding off, while the fresh sliced strawberries add bright color and juiciness. Crumbling the reserved mix over the top should create a patchy streusel that toasts beautifully. One pitfall is pressing too hard which yields a rock hard base; press firmly but gently to maintain a tender bite.
Bake for approximately 20-25 minutes, until the top is light golden brown and the jam is bubbling around the edges. Cool completely before cutting into bars.: During baking the kitchen fills with the scent of toasted oats and sweet strawberry, and you may hear a gentle bubbling as the jam warms. Visual cues are crucial here, the top should take on a pale golden hue and the jam should be actively bubbling at the edges, indicating it is fully heated through. Allowing the pan to cool completely sets the jam and keeps the bars from falling apart when sliced. A common mistake is cutting while still warm, which leads to messy, sagging squares instead of neat bars, so plan to wait until the internal temperature drops and the jam firms up slightly.