In a pot, bring 8 cups of water to a boil and cook potatoes for 10 minutes, or until tender.
Drain potatoes and cool in ice water.
Drain and set aside.
Slice chicken into bite-size pieces.
Set aside.
In a saucepan, combine oil, carrots, celery, mushrooms, onions and thyme.
Sauté on medium heat 10 minutes, or until vegetables are tender.
Set aside.
Melt margarine in a 5-quart pot.
Add garlic and sauté on medium heat for 2 minutes.
Stir in flour and cook on low heat for 5 minutes, stirring constantly.
Whisk in broth, salt and peppers.
Heat to a low boil on medium heat and simmer for 5 minutes, until thickened, stirring constantly.
Stir in milk and cream.
Simmer for 3 minutes.
Add peas, parsley, potatoes, vegetables and chicken to broth.
Season with salt and pepper and cook until heated through.
Place a buttermilk biscuit in a soup bowl.
Ladle stew over biscuit.