Step 1: For the Crust: In a large bowl, whisk together 1½ cups all-purpose flour, ¼ cup granulated sugar, and ½ teaspoon salt. Cut in the cold, cubed ½ cup unsalted butter using a pastry blender or your fingertips until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining.
Step 2: In a small bowl, whisk the large egg yolk with 2 tablespoons of ice water. Gradually add the egg mixture to the flour mixture, mixing until just combined. If the dough is too dry, add more ice water, 1 tablespoon at a time, until it comes together.
Step 3: Form the dough into a disc, wrap tightly in plastic wrap, and refrigerate for at least 30 minutes.
Step 4: For the Apple Filling: In a medium bowl, combine the diced apples, ¼ cup granulated sugar, 1 tablespoon all-purpose flour, ½ teaspoon ground cinnamon, ¼ teaspoon ground nutmeg, and 1 tablespoon lemon juice. Toss gently to coat all the apple pieces.
Step 5: For the Crisp Topping: In another medium bowl, combine ½ cup rolled oats, ¼ cup all-purpose flour, ¼ cup packed light brown sugar, and ¼ teaspoon ground cinnamon. Drizzle in the 2 tablespoons melted unsalted butter and mix with a fork until crumbly.
Step 6: Preheat oven to 375°F (190°C). Lightly grease a 24-cup mini muffin tin.
Step 7: On a lightly floured surface, roll out the chilled dough to about 1/8-inch thickness. Using a 2-inch round cookie cutter or glass, cut out 24 circles. Carefully press each dough circle into the cups of the prepared mini muffin tin, forming a small crust.
Step 8: Divide the apple filling evenly among the crusts, filling each about ¾ full.
Step 9: Sprinkle the oat crisp topping generously over the apple filling in each cup.
Step 10: Bake for 20-25 minutes, or until the crust is golden brown and the apple filling is bubbly. The topping should be nicely toasted.
Step 11: Let cool in the muffin tin for 10 minutes before carefully removing them to a wire rack to cool completely. Serve warm or at room temperature.