Sweet Potato Gratin with Orange and Ginger

1/2 stick butter (4 tablespoons), more for greasing pan
1 cup cream
2 tablespoons grated ginger
Zest from one orange
2 pounds sweet potatoes, peeled and grated, about 4 cups
1/2 cup chopped hazelnuts or walnuts (optional)
Salt and black pepper

Preheat oven to 400 degrees. Grease a 6- to 8-cup gratin dish with butter. In a small pan, gently heat remaining butter with cream, ginger and orange zest, stirring once or twice, until butter has melted and mixture is just steaming. Remove from heat.

Spread potatoes and nuts evenly into baking dish and sprinkle with plenty of salt and pepper. Drizzle cream mixture over all and press down on potatoes with a fork to settle them into dish and at least partly submerge them. Bake until potatoes are tender and cream thickens, about 45 minutes. Serve hot or warm.