For the crust and crumb
For the raspberry filling
Make the crust and crumb
Preheat the oven to 350°F. Butter the bottom and sides of a 9-by-13-inch glass or light-colored metal baking pan. Put a long piece of parchment paper in the bottom of the pan, letting the parchment extend up the two short sides of the pan and overhang slightly on both ends. (This will make it easy to remove the bars from the pan after they have baked.) Butter the parchment. Note: I used an aluminum foil "sling" as advised in America's Test Kitchen Family Cookbook.
Put the flour, brown sugar, oats, salt, baking powder, baking soda, and cinnamon in a food processor. Pulse in short bursts until combined. Add the butter and pulse until loose crumbs form.
Reserve 1 cup of the mixture and set aside. Pour the rest of the mixture into the prepared pan and use your hands or the back of a large wooden spoon to push the crust into an even layer at the bottom of the pan. The crust should touch the sides of the pan. Bake until golden brown, 12 to 15 minutes. Transfer to a wire rack and let the crust cool. Keep the oven on while you make the raspberry filling.
Make the raspberry filling
In a medium bowl, whisk the sugar, lemon zest, cinnamon and flour together. Add the raspberries, lemon juice and butter and use your hands to toss gently until the raspberries are evenly coated.
Assemble and bake the bars
Spread the raspberry filling evenly on top of the cooled crust. Sprinkle the reserved crust mixture evenly on top of the filling.
Bake for 35 to 45 minutes, rotating the pan every 15 minutes, until the top is golden brown and the filling starts to bubble around the edges.
Transfer to a wire rack to cool completely, then cut into squares and serve. The bars can be stored in the refrigerator in an airtight container for up to two days.
Makes 24 bars.