Adapted from Cook's Illustrated
2 1/2 cups flour
2/3 cup sugar
¼ teaspoon salt
16 tablespoons (2 sticks) plus 2 tablespoons butter, cut into 1/2-inch pieces, at room temperature
1/4 cup packed light brown sugar
1/2 cup quick oats
1/2 cup pecans, finely chopped
3/4 cup raspberry preserves
3/4 cup fresh raspberries
1 tablespoon fresh lemon juice
Preheat the oven to 375º. Line a 9 x 13-inch pan with non-stick foil so it hangs over the edges. Or use regular foil and spray with nonstick cooking spray.
Using mixer, combine the flour, sugar and salt at low speed to combine, about 5 seconds. With the machine on low, add 1 cup (2 sticks) of the butter, 1 piece at a time, then continue mixing on low for about 1 to 1 1/2 minutes.
Reserve 1 1/4 cups of the flour mixture and set aside. Evenly pat the remaining flour mixture in the bottom of the prepared baking pan. Bake until the edges begin to brown, 14 to 18 minutes. Leave oven on.
While the crust is baking, add the brown sugar, oats and pecans to the reserved flour mixture and mix to combine. Using your fingers pinch the remaining 2 tablespoons of butter into the flour and oat mixture until the butter is fully incorporated. Pinch the mixture with fingers to create small clumps; set streusel aside.
Mix preserves, raspberries, and lemon juice in a small bowl, mashing the berries into small pieces.
Using a spatula, spread the filling evenly over the hot crust, then sprinkle the streusel topping evenly over the filling without pressing down on it. Bake 22 to 25 minutes till topping is lightly browned and filling is bubbling.. Cool to room temperature on a wire rack, 1 to 2 hours; remove from the baking pan by lifting the overhanging foil. Cut into squares and serve.