1) Soak the mushrooms in a cup of hot water for 10 minutes. Meanwhile, heat the oil in a large frying pan over a medium-high heat.
2) Season the chicken with salt and pepper and add to the hot pan. Sear for 2 minutes on each side, until golden brown. Remove the chicken from the pan and set aside.
3) To the same pan, add the garlic and shallots and saute for 3 minutes, until soft. Add the sherry and thyme and cook for another minute.
4) Return the chicken to the pan with the chicken stock and the chestnuts. Drain the mushrooms through a paper towel-lined sieve. Add the mushrooms to the pan with the strained soaking liquid. Simmer for 5 minutes, or until the chicken is cooked through.
5) Meanwhile, cook the orzo pasta according to the directions on the packet.
6) Remove two of the chicken breasts and reserve for another meal. Add the double cream to the pan and simmer for 2 minutes.
7) Strain the pasta. Serve the chicken and porcini-chestnut sauce over the orzo pasta, with the steamed spinach on the side.