How to Make It
Roast the poblanos directly over a gas flame or under a preheated broiler, turning, until charred all over. Transfer to a bowl, cover tightly with plastic wrap and let cool. Peel, seed and stem the poblanos, then thinly slice them.
In a large skillet, heat the olive oil. Add the onion and cook, stirring, until golden, about 5 minutes. Add the butter, sweet potatoes, garlic and rosemary and cook over moderate heat, stirring occasionally, until the potatoes are tender and lightly browned, about 10 minutes. Stir in the poblanos and lemon juice and season with salt and pepper. Discard the rosemary sprig and serve.