Preheat the oven to 400 degrees F (200 degrees C).

Peel, rinse, and cut potatoes in half lengthwise. Cut each half into 3 strips, then cut across into 3/8-inch slices. Transfer slices into a large bowl of cold water and let sit for a few minutes to rinse off some of the starch. Transfer into a colander and rinse well; drain very well.

Transfer potatoes to a 13x9-inch casserole dish. Sprinkle with salt, white pepper, and cayenne pepper; pour broth over and toss thoroughly with a spoon. Press everything down into an even layer.

Bake in the preheated oven until potato slices are just tender, about 1 hour.

Meanwhile, add breadcrumbs and butter to a small bowl; stir until the mixture resembles wet sand. Set aside. Mix milk and cream together; set aside.

Remove potatoes from the oven and turn oven temperature up to 450 degrees F (235 degrees C).

Pour cream mixture over potatoes, and toss thoroughly with a spoon. Spread out again into an even layer. Scatter cheese evenly over the top, followed by breadcrumbs.

Bake potatoes until browned and bubbling around the edges, about 20 minutes more.

Let stand for 15 minutes. Sprinkle with sliced green onions before serving.