Preheat your oven to 375°F. Bring a large pot of salted water to a rolling boil. Carefully add sliced sweet potatoes and cook for 10-12 minutes until just tender when pierced with a fork but still holding their shape. Drain thoroughly and let them cool slightly.

Grease a 9x13-inch baking dish with butter or cooking spray. Arrange half the sweet potato slices in overlapping layers across the bottom, overlapping by about half. Scatter half the cranberries evenly over the potatoes.

In a medium bowl, whisk together heavy cream, brown sugar, cinnamon, salt, and pepper until the sugar dissolves completely. Pour half this mixture over the layered sweet potatoes and cranberries, letting it seep between the layers.

Create a second layer with remaining sweet potatoes and cranberries. Pour the rest of the cream mixture over the top, gently pressing down so the liquid reaches all layers.

Sprinkle shredded Gruyère cheese evenly across the top, followed by breadcrumbs. Bake for 30-35 minutes until the top is deeply golden brown and bubbling around the edges. Let rest for 5-10 minutes before serving to allow the cream to thicken slightly.