Step 1Heat 2 tablespoons olive oil in a large heavy stockpot on medium-low heat. Add the celery, onions, garlic, 2 tablespoons thyme and white pepper and saute over low heat until the vegetables are nearly tender, about 10 minutes.
Step 2Reserve eight cremini mushrooms (to use later for garnish), then mince the remaining cremini and button mushrooms in a food processor.
Step 3Add the mushrooms to the pot and continue to saute until all the vegetables are tender, about 5 minutes.
Step 4Add the Port, bring to a boil, then reduce the heat and simmer uncovered for 30 minutes. Add the chicken stock, bring to a boil, then reduce the heat and simmer 30 more minutes. Add the cream and cook 15 more minutes.
Step 5Puree the soup in the same pot using a hand-held blender or remove to a blender, process until smooth and return to the pot.
Step 6In a small pan, melt the butter and whisk in the flour to make a paste to thicken the soup. Add to the pureed mixture, stirring until completely blended. Bring the soup to a simmer again, then reduce heat and continue to cook on low heat for 15 minutes until thickened, stirring often so as not to burn bottom of the pan.
Step 7Slice the reserved cremini mushrooms. Heat the remaining 1 tablespoon of olive oil in a small saute pan. Add the sliced mushrooms and saute until golden brown, about 5 minutes. Use to garnish the soup along with the remaining 1 tablespoon thyme leaves.