Cook chicken pieces with tablespoon oil in pan and brown on both sides. Remove from pan and drain on paper towel.
Remove any excess fat from pan, add 1 tbsp of butter and cook potatoes, sweet potato, celery and carrots for 1 to 2 minutes and remove.
Cook mushrooms for 1 minute in 1 tbsp butter, remove.
Add 2 tbsp butter to pan with 1 tbsp oil, add sliced leeks and garlic and cook for 1 to 2 minutes until soft. Remove.
Add 2 tbsp butter to pan, add flour and cook for 1 minute or until butter is absorbed. Slowly add chicken stock stirring between additions until you have a smooth gravy . Add additional stock or water if too thick. Add thyme and cook for 1 minute.
Put all ingredients into slow cooker or casserole dish and mix well. Slow cook for 6 hours or until meat is thoroughly cooked. If cooking in oven, chicken should be cooked in 1 to 1 1/2 hours.
Serve as is or with mash or noodles.
If you are cooking this in the oven, you may want to reduce this recipe in half to accommodate standard casserole dishes. I have a hungry family to feed so we cook enough for two dinners.