- 6 Tbsp. butter, divided, plus more for greasing
- 3 large Granny Smith apples (about 1½ pounds), peeled, cored, and cut into ⅓-thick slices
- 2 large sweet potatoes (about 2¼ pounds), peeled and cut crosswise into ⅓-thick rounds
- 2 cups heavy cream (or half-and-half or milk)
- 1½ tsp. nutmeg
- 1½ tsp. kosher salt
- ¾ tsp. cinnamon
- ½ tsp. ground black pepper
- ¾ cup fresh bread crumbs
Active time: 30 minutes
Total time: 1½ hours
Preheat oven to 375°. Grease a 9" × 13" baking dish with butter; set aside. In a large skillet over medium-high heat, melt 3 Tbsp. butter. Add apples and cook, stirring occasionally, until just caramelized, about 10 minutes. Meanwhile, in a large bowl, mix sweet potatoes with heavy cream, nutmeg, salt, cinnamon, and pepper.
Transfer half of sweet potatoes to prepared dish, arranging them in an overlapping pattern like shingles. Cover with apples, then remaining sweet potatoes, overlapping the potatoes as before. Pour any remaining cream over the top. Cover with foil and bake until tender, about 45 minutes.
Increase oven temperature to 500°. Uncover dish, scatter bread crumbs over the top, and dot with remaining 3 Tbsp. butter. Bake, keeping a close eye on the gratin in case bread crumbs start to burn, until golden brown on top, 8 to 10 minutes more. Serve immediately.