Meanwhile, heat oil in a large skillet over medium-high heat. Season chicken with salt and pepper and add to hot pan. Sear 2 minutes on each side, until golden brown. Remove chicken from skillet and set aside. To the same pan, add garlic and shallots and saute 3 minutes, until soft. Add sherry and thyme and cook 1 minute. Return chicken to pan with the chicken broth and chestnuts. Drain mushrooms through a paper towel-lined sieve and add to pan with the strained soaking liquid. Simmer 5 minutes, until chicken is cooked through. Remove 2 chicken breast halves and reserve for another meal. To the skillet, add heavy cream and simmer 2 minutes. Remove from heat and serve chicken and mushroom sauce over half of the orzo pasta, reserving second half of orzo for another meal. Serve spinach on the side.