In the same skillet, melt 2 tablespoons of the butter. Add the white mushrooms and cook over moderately high heat until browned in spots, about 5 minutes. Add the onion, carrots and celery and cook over moderate heat until tender, about 5 minutes. Add the porcini and season with salt and pepper. Sprinkle in the flour and cook, stirring, for 2 minutes. Return the chicken and any juices to the skillet. Stir in the chicken stock, thyme sprigs and the porcini soaking liquid and bring to a simmer over moderate heat, scraping up any bits from the bottom of the skillet. Simmer until the sauce has thickened and the chicken is cooked through, about 10 minutes. Spoon the chicken into a medium baking dish; discard the thyme sprigs.