Apple Oat Bars with Pretzel Crumb Topping

1. Preheat oven to 350ºF with a rack in the middle position. Grease a glass 9×13″ baking dish.
2. For the crust: In a medium bowl, combine brown sugar, baking powder, oats, and flour. Cut butter into half-inch pieces. Add butter to the oat mixture, working it in until the dry ingredients are moistened. (You can do this with a wooden spoon or in a food processor, or with your hands—my favorite.) Stir in the beaten egg.
3. Transfer crust mixture to prepared baking dish, pressing it out evenly to the sides and corners of the dish. Set aside.
4. For the filling: Place chopped apples, flour, lemon juice, cinnamon, and granulated sugar in a large bowl and toss to combine. Pour filling over the crust and spread evenly.
5. For the crumb topping: In a medium bowl, combine the crushed pretzels, flour, brown sugar, and granulated sugar. Cut in the butter (like we did in step 2) until the mixture has a crumbly texture, with pieces about the size of a blueberry.
6. Sprinkle the crumb topping evenly over the filling. Bake until top is golden brown and apples are tender, about 40 minutes.
7. Cool bars in the pan on a wire rack until completely cool. Slice and serve.