1 3/4 cups walnut or pecan halves
2 1/3 cups brown sugar
3 cups rolled oats, divided
1 cup all-purpose flour
1 cup whole-wheat or spelt flour
1 1/2 teaspoons kosher salt
1 teaspoon baking powder
16 tablespoons (8 ounces) unsalted butter, cut into small pieces and softened at room temperature
2 cups fruit jam (any flavor)
Preheat the oven to 350ºF and line a 9x13-inch baking dish with a sling of parchment paper.
In a food processor, briefly pulse the walnut halves until coarsely chopped. Place in a large mixing bowl and mix in the brown sugar and 2 cups of the oats. (See Recipe Notes for making this recipe without a food processor.)
In the now-empty food processor, combine the remaining 1 cup of oats with the all-purpose and whole-wheat flours, salt, and baking powder. Process in brief pulses until oats are pulverized, about 10 one-second pulses. Add the butter to the food processor and pulse briefly until the butter is fully incorporated, about 5 one-second pulses.
Combine the oat-butter mixture with the chopped walnuts, brown sugar, and oats in the mixing bowl. Use a dough whisk or sturdy long-handled spoon to mix until the ingredients are thoroughly blended — the mixture will still be loose and crumbly, but should hold together if you press a little in your fist. You can also complete this step in a stand mixer with a paddle attachment, if desired.
Transfer half of the granola mixture to the lined baking dish and press it down firmly, making sure to spread it out evenly all the way to the edges and corners of the dish. Dollop the jam on top, then spread it into an even layer with an offset spatula or the back of a spoon. Sprinkle the second half of the granola mixture over the top of the jam in an even layer, patting it gently into the jam to create a flat surface.
Bake the bars just until the top is lightly browned and the jam has begun to bubble up around the edges, about 35 minutes. Rotate the baking dish once during cooking.
Remove bars from oven and allow to cool completely in the pan, about 3 hours. Slice into 2x3-inch bars and serve, or store in a tightly lidded container for up to one week.