Put chicken, stock water, onion, celery and bay leaf in an 8-quart pot. Bring to a gentle simmer and cook partly covered for 75 to 90 minutes until the chicken reaches 165°F in the breast and innermost thigh and lifts easily from the bones.
Meanwhile, bring brown rice and rice water to a boil in a small saucepan. Cover and cook on low for 40 to 45 minutes, then rest covered off heat for 10 minutes.
Lift chicken onto a tray. Strain the stock, discard spent vegetables and bay leaf, skim excess fat and measure 8 cups, adding water if needed. Pull chicken from the bones, discard skin and bones and tear the meat into small pieces.
Heat oil in the empty soup pot and soften leeks for 5 minutes. Add parsnips, carrots, thyme, 1/2 teaspoon salt, pepper and measured stock; simmer for 15 to 20 minutes until vegetables are tender.
Add all the chicken meat and spinach, simmer for 3 minutes until the chicken reaches 165°F, then taste and add up to the remaining salt. Divide cooked rice among eight shallow freezer containers and add equal amounts of soup.
Cool promptly, refrigerate within 2 hours and chill completely. Cover with expansion space and freeze for up to 3 months for best quality.
To reheat one frozen portion, loosen it from the container into a saucepan with 2 tablespoons water. Cover over low heat until it releases liquid, then break it apart, stir frequently and bring to a full simmer with all chicken pieces at 165°F, usually 15 to 25 minutes.