3 pounds sweet potatoes, peeled and sliced 1/2 inch thick
3 tablespoons unsalted butter, melted
3/4 teaspoon coarse sea salt
6 medium poblano chiles
3 small onions, sliced crosswise 1/4 inch thick, separated into rings
1 cup (loosely packed) cilantro leaves
1/3 cup crème fraîche or sour cream
Preheat the oven to 375°. Generously butter a 3-quart gratin dish. Arrange the sweet potatoes in an overlapping layer in the prepared dish. Brush the potatoes with 1 1/2 tablespoons of the melted butter and sprinkle with the salt. Bake for about 40 minutes, or until tender and just beginning to brown.
Roast the poblanos directly over a gas flame or under the broiler, turning, until charred all over. Transfer the poblanos to a bowl, cover with plastic and let steam for 15 minutes. Peel, core and seed the chiles, then cut them into 1/4-inch strips.
Transfer the poblanos to a bowl, add the onion rings and toss with the remaining 1 1/2 tablespoons of butter. Scatter the poblano mixture over the sweet potatoes.
Reduce the oven temperature to 350° and roast the sweet potatoes for about 35 minutes, or until the poblanos and onions are tender and just beginning to brown. Scatter the cilantro leaves on top, drizzle with the crème fraîche and serve.
The sweet potatoes can be prepared through Step 3 and refrigerated overnight. Let return to room temperature before baking.