1. Make the crust: Preheat oven to 375 degrees. Butter a 9-by-9-inch baking pan. Line with parchment paper, leaving a 2-inch overhang. Butter parchment; set aside.

2. In the bowl of a food processor, combine flour, sugar, salt, and butter. Pulse to combine. Add egg and pulse until evenly moistened. Transfer mixture to prepared pan; press into bottom (use the base of a dry measuring cup or a small offset spatula to help). Freeze for 10 minutes. Bake until lightly golden, 20 to 25 minutes.

3. Make the filling: In a saucepan, combine dates, sugar, and cider. Cook over medium heat until soft and the consistency of jam. Using a hand blender, puree mixture until smooth. Stir in lemon zest. Spread mixture evenly over baked crust.

4. Make the topping: In a medium bowl, whisk together oats, flour, brown sugar, salt, and cinnamon. Using your hands, work in butter to form crumbs. Evenly distribute crumbs over filling to cover. Bake until topping is golden brown and filling is set, 20 to 25 minutes. Run a paring knife around edges of pan to loosen; let cool completely in pan on wire rack. Using parchment overhang, remove from pan; cut into squares using a serrated knife.