Season the chicken thighs evenly with the Ranch salad dressing and seasoning mix.
In a large, heavy-bottomed saucepan, heat the oil over high heat. Add the chicken thighs, skin side down, and cook undisturbed until browned, about 5 minutes. Flip over and cook another 3–4 minutes. Remove from pan and set aside.
Turn heat down to medium, and add the mushrooms. Sauté until mushrooms are slightly softened, about 3–4 minutes. If the bottom of the pan looks as if it might start to burn, stir in a tablespoon or two of water.
Add chicken thighs back into the pot, and pour in the chicken broth. Bring liquid to a boil, then cover, turn heat down to low, and simmer for 30 minutes. Remove cover, stir in ranch dressing, and simmer for another 5 minutes, or until chicken is fully cooked and liquid has slightly reduced. Garnish with parsley and serve.
For safe meat preparation, reference the USDA website.