1. Peel sweet potatoes and cut crosswise into 1/4-inch-thick slices. Cut unpeeled orange in half through stem, then crosswise into 1/4-inch-thick slices, discarding end pieces and seeds.
2. In a large bowl, mix 1/3 cup marmalade, 1/2 cup brandy (1/3 cup if using yams), butter, ginger, and 1/2 teaspoon salt. Add sweet potatoes and mix to coat. Arrange slices in rows in a single layer, overlapping them slightly, in a shallow 3-quart casserole (such as a 9- by 13-in. baking dish), interspersing the orange slices evenly throughout. Drizzle any of the remaining brandy mixture over the sweet potatoes. Cover tightly with foil.
3. Bake in a 325° regular or convection oven (350° if baking with a turkey at that temperature) until sweet potatoes are tender when pierced, 50 to 60 minutes.
4. In a small bowl, mix remaining 1/3 cup marmalade and 1 tablespoon brandy. Uncover sweet potatoes and brush marmalade mixture evenly over the top. Broil 6 inches from heat until lightly browned, 8 to 10 minutes. Add more salt to taste.