Heat the remaining 1 tablespoon of olive oil in the casserole. Add the onions and cook over low heat until softened, about 10 minutes. Add the garlic and cook, stirring, until fragrant, about 2 minutes. Add the chile powder and cook, stirring, for 4 minutes. Add the tomato paste mixture, chicken stock, ancho and porcini. Add the ancho and porcini soaking liquids, stopping when you reach any grit at the bottom. Season with a large pinch of salt and simmer over moderately low heat until the sauce reduces by a third, about 10 minutes. Gently stir in the meat and any accumulated juices and season with salt and pepper. Rewarm just before serving.