Step 1Melt the butter in a 4 1/2--quart pan over low heat. Add the onion, celery, garlic, thyme and mushrooms, then saute till onions are translucent, about 5 minutes, stirring often.
Step 2Add the artichoke hearts and cook for 3 minutes until heated through.
Step 3Add the Sherry and Brie and continue to cook, stirring often, for 2 minutes, until the Brie melts.
Step 4Add the chicken stock and bring to a boil.
Step 5Make a roux by stirring the butter and flour together in a small saucepan and cooking over low heat for about 8 minutes. Add the roux to the soup slowly to prevent lumps from forming and cook for 10 minutes until the soup thickens.
Step 6Add the cream and white pepper and simmer for 5 minutes more.