This is one of the world's most forgiving desserts, and it's always a crowd-pleaser. The streusel acts as a crumbly press-in crust as well as a topping.
Lots of substitutions will work here: You can use spelt flour in place of the whole wheat (or use only all-purpose flour); use a different fruit jam; or try another nut. You can also try adding various spices. Take this recipe and make it yours.
Preheat oven to 350°F. Butter a 9- by 13-inch baking pan and line it with parchment paper so that the paper overhangs the long sides of the pan by about 1 inch.
In a large mixing bowl, stir together whole wheat flour, all-purpose flour, oats, granulated sugar, brown sugar, salt, and baking soda. Pour 1 cup melted butter over streusel mixture and stir until ingredients are evenly moistened.
Firmly press all but 3/4 cup of streusel in an even layer into prepared baking pan. Spread jam on top, leaving a 1/4-inch border all around. Press remaining streusel into clumps, add walnuts, and toss. Sprinkle walnut streusel all over jam.
Bake until top is golden brown, about 40 minutes. Transfer to a wire rack and let cool completely, about 2 hours.
Using overhanging parchment paper, lift dessert from baking pan and transfer to a work surface. Cut into bars. Bars can be stored at room temperature for up to 3 days.
Special Equipment
9- by 13-inch baking pan, wire rack