In a large deep skillet, melt the butter until foaming. Add the onions, celery, carrots and garlic and cook over moderately high heat, stirring, until softened, 8 to 10 minutes. Add the sausage, breaking it up, and cook until no pink remains, about 10 minutes. Stir in the oregano, salt, pepper and poblanos and transfer to a large bowl. Mix in the corn bread, then stir in the turkey stock. Spread the stuffing in the prepared gratin dish.