Heat the oven to 350°F (175°C). Arrange the frozen mini phyllo shells on a parchment-lined baking sheet, keeping them frozen until they are ready to fill.

Add the finely diced apples, 1 tablespoon butter, 2 tablespoons brown sugar, lemon juice, cornstarch, 1/2 teaspoon cinnamon, optional nutmeg, and a pinch of salt to a small saucepan. Cook over medium heat for 4–5 minutes, stirring often, until the apples begin to soften and the juices thicken; cool slightly.

For the topping, combine the oats, flour, 1/4 cup brown sugar, 1/4 teaspoon cinnamon, salt, and optional chopped nuts in a bowl. Rub or cut in the cold butter until coarse crumbs form.

Spoon a small amount of the warm apple filling into each frozen phyllo shell. Fill neatly without overfilling, since too much filling can spill over as the bites bake.

Sprinkle the oat topping over each filled shell. Use just enough to cover the apples while keeping the small appetizers easy to pick up.

Bake for 15–18 minutes, until the phyllo shells are crisp and the streusel is lightly browned. Watch closely near the end because the delicate shells can brown quickly.

Let the apple crisp bites cool on the baking sheet for several minutes before serving. Offer them plain or with caramel sauce, whipped cream, vanilla ice cream alongside, or a light dusting of powdered sugar.