Place the tart shells on a non-stick or parchment paper-lined baking sheet. Preheat the oven to 350 degrees F. Combine the butter, oats, pecans and brown sugar in a medium bowl. Place the lemon juice in a second medium bowl. Add the apples and toss to coat. Add the granulated sugar, apple juice, cornstarch, cinnamon and nutmeg and mix to combine.
Spoon and mound the apple mixture into the tart shells, ensuring all the liquid in the bowl makes its way into the shells. Divide and top the apples with the oat mixture, lightly packing it on. (The tarts will look quite mounded, but will settle down once the apples cook and soften.) Bake the tarts 30 to 35 minutes, or until golden brown. Serve at room temperature or, refrigerate tarts, and serve them chilled, removing the foil liners first, if desired.
Recipe Options: Make walnut crumble topping by replacing the pecans with an equal amount of chopped walnuts. Make pear tarts by replacing the apples with three, small to medium ripe pears, peeled, cored and cubed.