Preheat your oven to 350°F (175°C) and line your baking pan with parchment paper for easy removal.
In a mixing bowl, combine the flour and sugar for the crust and crumb topping. Mix just until evenly blended.
Add the cold, small pieces of butter into the flour mixture. Using your fingertips or a pastry cutter, cut in the butter until the mixture resembles coarse crumbs with some pea-sized pieces.
Press about two-thirds of this crumb mixture firmly into the bottom of the prepared baking pan, covering the entire surface evenly. This forms the crust.
Spread the strawberry jam over the crust layer, smoothing it out with the back of a spoon or spatula to create a vibrant, even filling.
In a small bowl, whisk together the lemon zest, lemon juice, and the remaining crumb mixture to make a lemon-infused crumb topping.
Sprinkle the remaining crumb mixture evenly over the jam layer, ensuring some of the jam shows through for visual contrast.
Bake in the preheated oven for 40 minutes, or until the topping is golden brown and bubbling around the edges.
Remove from the oven and allow the bars to cool completely in the pan on a wire rack. This helps the filling set and makes slicing easier.
Once cooled, cut into squares and serve. These bars are best enjoyed fresh but can be stored in an airtight container for a few days.
