2 1/2 cups unbleached all-purpose flour
2/3 cup granulated sugar
1/2 teaspoon table salt
16 tablespoons unsalted butter (2 sticks) plus 2 tablespoons, cut into 1/2-inch pieces and softened to cool room temperature
1/4 cup packed brown sugar, light or dark
1/2 cup rolled oats
3/4 cup raspberry preserves
3/4 cup fresh raspberries
1 tablespoon lemon juice from 1 lemon
1/2 cup pecans, chopped fine (optional)
1. Adjust oven rack to middle position; heat oven to 375 degrees. Cut 18-inch length foil and fold lengthwise to 8-inch width. Fit foil into length of 13 by 9-inch baking dish, pushing it into corners and up sides of pan; allow excess to overhang pan edges. Cut 14-inch length foil and fit into width of baking pan in same manner, perpendicular to first sheet. (If using extra-wide foil, fold second sheet lengthwise to 12-inch width.) Spray foil-lined pan with nonstick cooking spray.
2. In bowl of standing mixer fitted with flat beater, mix flour, granulated sugar, and salt at low speed until combined, about 5 seconds. With machine on low, add 16 tablespoons butter one piece at a time; then continue mixing on low until mixture resembles damp sand, 1 to 1 1/2 minutes. (If using food processor, process flour, granulated sugar, and salt until combined, about 5 seconds. Scatter 16 tablespoons butter pieces over flour mixture and pulse until mixture resembles damp sand, about twenty 1-second pulses.)
3. Measure 1 1/4 cups flour mixture into medium bowl and set aside; distribute remaining flour mixture evenly in bottom of prepared baking pan. Using hands or flat-bottomed measuring cup, firmly press mixture into even layer to form bottom crust. Bake until edges begin to brown, 14 to 18 minutes.
4. While crust is baking, add brown sugar, oats, and nuts to reserved flour mixture; toss to combine. Work in remaining 2 tablespoons butter by rubbing mixture between fingers until butter is fully incorporated. Pinch mixture with fingers to create hazelnut-sized clumps; set streusel aside.
5. Combine preserves, raspberries, and lemon juice in small bowl; mash with fork until combined but some berry pieces remain.
6. Spread filling evenly over hot crust; sprinkle streusel topping evenly over filling (do not press streusel into filling). Return pan to oven and bake until topping is deep golden brown and filling is bubbling, 22 to 25 minutes. Cool to room temperature