Thinly slice onions or dice and cook over medium heat in 2 tablespoons olive oil until mushy but not caramelized (approx. 15 minutes). Add celery root and cook 5 additional minutes. To the onion and celery root mixture, now add the roasted garlic and mushrooms and cook until the mushrooms are soft. Add fresh thyme, and season to taste. Add stock, bay leaf and cover and simmer for 30 minutes over a low medium heat. Add heavy cream, tamari sauce and 1 teaspoon lemon juice and cook for 5 minutes more.
Place the ½ cup mushrooms that have been reserved and fry in 1 tablespoon butter until mushrooms nicely browned. Set aside for garnish. The soup can be left as is for a more rustic presentation or pureed.
Ladle soup into bowls and garnish with chopped basil, fried mushrooms and a piece of rye toast.
Note: if using soy sauce instead of tamari sauce, reduce the salt used for seasoning.