Preheat oven to 350 degrees Fahrenheit.

Peel and thinly slice the sweet potato.

Peel, core, and cut the apples into 1/4 inch-thick slices.

Place the apple slices in a bowl and cover with water; add the lemon juice.

Trim the leek, discarding the top 1 inch and the root end.

Cut the leek lengthwise in half and clean thoroughly under cool running water.

Thinly slice the leek crosswise.

Drain the apples well and pat them dry.

Grease a 1 1/2-quart casserole dish with 1 tablespoon of butter.

Place one-third of the apple slices in the bottom of the casserole dish; top with one-third of the leek slices and one-third of the sweet potato slices.

Season with salt and pepper.

Repeat the layering process two more times.

Dot the top of the casserole with 2 tablespoons of butter and pour apple juice evenly over all.

Cover the casserole tightly with a lid or aluminum foil and bake for 45 minutes.

While the casserole is baking, prepare the topping by heating the remaining 1 tablespoon of butter in a small saucepan.

Stir in the bread crumbs and brown sugar until well combined.

Uncover the casserole and sprinkle the topping evenly over the surface.

Bake uncovered for an additional 10 to 15 minutes, or until the potato slices are tender.

Serve the casserole immediately.