Preheat oven to 350 degrees Fahrenheit (175 degrees Celsius). Line an 8x8″ baking pan with foil or parchment paper.
In a medium mixing bowl, whisk together the flour, ⅓ cup granulated sugar, and salt. With a pastry blender, cut in 8 tablespoons of butter until the mixture resembles wet sand. Reserve half of the mixture for the topping.
Sprinkle half of the mixture into the prepared pan and press into an even layer. Bake the crust until the edges begin to brown, about 15-17 minutes. Remove and allow to cool for 10 minutes.
Meanwhile, combine the reserved flour mixture, oats, nuts, and brown sugar together in a medium bowl. Add the remaining 2 tablespoons of butter and mix until small clumps begin to form. Set aside.
In a small bowl, using a rubber spatula, combine the jam, raspberries, sugar, cornstarch, vanilla, lemon juice, and zest.
Spread the berry mixture evenly over the warm crust, then sprinkle with the streusel topping. Place in the oven and bake until the filling is bubbling and the topping is golden brown, about 24 to 26 minutes, rotating the pan halfway through baking.
Remove from the oven and let the bars cool completely in the pan on a wire rack for about 1 hour. Remove the bars from the pan using the foil, cut into squares, and serve.
Notes
STORE the Raspberry Bars in an airtight container, such as Tupperware, at room temperature for up to 5 days. For a firmer texture, keep them in the fridge, which helps the crust hold together better.To FREEZE, place them in the freezer, bake the bars, let them cool completely, and place them in an airtight container or bag. Store them in the freezer for up to 3 months. When ready to eat, thaw overnight in the fridge, and enjoy!