Preheat oven to 400°F (200°C). Line a large baking sheet with parchment paper or a silicone baking mat.

In a small bowl, stir together the brown sugar, cinnamon, and nutmeg. Set aside.

In a large bowl, toss the thinly sliced apples with lemon juice. Add the reserved sugar mixture and toss until the apples are evenly coated.

Unroll the pie crust onto the prepared baking sheet. Gently roll it out slightly to form a 12 to 13-inch circle.

Arrange the apple slices in the center of the pie crust circle, leaving a 2-inch border around the edges. Try to keep the apples mounded slightly in the center.

Gently fold the edges of the pie crust up over the apples, overlapping as necessary to create a rustic, free-form tart. In a small bowl, whisk together the egg and milk for the egg wash, then brush it over the exposed crust.

Bake for 28-32 minutes, or until the crust is golden brown and the apples are tender. If the crust begins to brown too quickly, you can loosely tent it with aluminum foil.

Let the apple tart cool to room temperature on the baking sheet before serving. It can be served plain, warm, or with a scoop of vanilla ice cream. Store any leftovers in the refrigerator.