Combine sliced apples, lemon juice, lemon zest, granulated sugar, cinnamon, nutmeg, and salt in a medium bowl. Stir to coat evenly. Cover and refrigerate for at least 1½ hours, stirring occasionally.
Preheat the oven to 350°F with the rack in the middle.
In a medium bowl, stir together brown sugar, flour, and oats. Add cold butter cubes and toss to coat. Work the butter into the dry ingredients by hand, squeezing and mashing until the mixture clumps together with only a few remaining butter pieces. Work quickly to keep the butter cold.
Give the apples one final stir and pour into an 8-inch square baking dish. Cover with the streusel topping, pressing some down into the apples. Use as much or as little as you like.
Bake for 45 to 50 minutes, until the topping is deep golden brown and the filling is bubbling at the sides. For very soft apples, bake an additional 10 to 15 minutes.
Cool on a wire rack for 10 minutes. Serve warm with vanilla ice cream.
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Notes
Maceration: Letting the apples sit in the sugar for at least 1½ hours draws out moisture and produces a saucy, bubbling filling. Skipping this step is the most common reason apple crisp comes out dry. Doneness: The crisp is ready when the topping is deep golden brown and the filling is actively bubbling out at the sides. If only the top is browned, keep baking. Apples: Use tart varieties like Granny Smith, Cripps Pink, or Honeycrisp. Avoid Red Delicious, which turns to mush. A mix of two or three varieties gives the best flavor and texture. Storage: Cool completely, wrap well in plastic wrap, and refrigerate for up to one week or freeze for up to two months. Reheat in a 350°F conventional oven until warmed through. Do not use convection, as it will scorch the topping before the filling heats.