Place sweet potatoes in a large pot of water and bring to a gentle boil.

Parboil (partially cook) for about 20 minutes, or until the potatoes can be pierced with a fork but are still firm in the center — they’ll finish cooking in the oven.

Drain, let cool slightly, then peel and cut into thick slices (about ½–¾ inch).

Peel, core, and slice apples into even slices.

In a small saucepan, melt butter over medium heat.

Stir in sugar, cornstarch, water, and salt.

Cook, whisking constantly, until the mixture comes to a gentle boil and thickens slightly. Once it starts to boil, remove from heat. The sauce should look glossy and smooth, with a syrupy consistency.

Preheat oven to 350 degrees.

Butter or spray with non-stick cooking spray a 2-quart baking dish (an 8x8-inch square dish or a medium oval casserole works well).

Arrange half of the sweet potato slices on the bottom, followed by half of the apple slices.

Repeat the layers with the remaining sweet potatoes and apples.

Slowly pour the warm sauce evenly over the top so it seeps down between the layers.

If using cinnamon, sprinkle with cinnamon.

Bake in a 350°F oven for 45 minutes to 1 hour, or until the apples are tender and the sauce is bubbling around the edges.

If the top browns too quickly, loosely cover with foil for the final 10–15 minutes.

Let it rest for about 10 minutes before serving — the sauce will thicken slightly.