FOR THE FILLING
4 pounds apples (8 or 9, assorted varieties), peeled, cored, and cut into ½-inch slices
¼ cup granulated sugar
1 tablespoon fresh lemon juice
2 teaspoons ground cinnamon
¼ teaspoon finely grated fresh ginger
2 tablespoons quick-cooking tapioca or all-purpose flour
⅛ teaspoon kosher salt

FOR THE CRUMBLE TOPPING
1 ¼ cups all-purpose flour
1 ¼ cups old-fashioned rolled oats
1 ¼ cups firmly packed dark brown sugar
½ teaspoon ground cinnamon
⅛ teaspoon kosher salt
12 tablespoons (1 ½ sticks) unsalted cold butter, cut into small cubes