Step 1

Place a rack in middle of oven; preheat to 350°. Butter a 10" cast-iron skillet or a 2½-qt. shallow round or oval baking dish. Melt 6 Tbsp. butter in a small saucepan over medium heat and cook, stirring often, until butter foams, then browns (do not burn) and smells nutty, 5–8 minutes.

Step 2

Remove saucepan from heat and whisk in garlic, ginger, orange zest, orange juice, broth, brown sugar, soy sauce, miso, salt, five-spice powder, pepper, and 1 Tbsp. thyme.

Step 3

Combine sweet potatoes and brown butter mixture in a large bowl and toss gently until sweet potatoes are thoroughly coated.

Step 4

Stack sweet potato slices upright on their edges in vertical rows along perimeter of skillet, following the curve of the pan. Arrange more vertical rows of sweet potatoes down center of skillet to cover entire pan. Be sure to pack sweet potatoes tightly and fill in any gaps with smaller sweet potato slices. Pour any remaining brown butter from bowl over sweet potatoes, then cover skillet tightly with foil.

Step 5

Bake sweet potatoes until fork-tender, 50–60 minutes. Carefully remove foil and increase oven temperature to 425°. Continue to bake until tops of sweet potatoes are deep golden brown, 30–40 minutes longer.

Step 6

Scatter more thyme over tian and let cool slightly before serving.Do Ahead: Tian can be baked 1 day ahead. Let cool; cover tightly with foil and chill. Reheat, loosely covered with foil, in a 350° oven until heated through, 40–45 minutes.