Meanwhile, in a large nonreactive saucepan, melt the 3 tablespoons butter over moderate heat. When the foam subsides, add the shallots and reduce the heat to low. Cook, stirring a few times, until softened, about 5 minutes. Increase the heat to moderately high and add the cremini mushrooms. Season with salt and pepper and cook, stirring, until wilted, about 4 minutes. Add the wine and boil until reduced by half, about 3 minutes. Stir in the flour until well blended. Gradually whisk in the Rich Chicken Stock and bring to a simmer. Reduce the heat to low, cover and cook until the mushrooms are very tender, about 15 minutes.