Melt butter in a large heavy-bottomed saucepan. Add leeks and cook until tender, stirring occasionally, about 5 minutes. Add garlic and thyme, and cook 1 minute, stirring often. Add mushrooms, and cook until mushrooms are cooked through and golden, about 5 minutes.

Slowly add flour, while stirring continuously. Add stock and bring to a boil.

Reduce heat, and simmer 10 minutes.

Puree about 1/3 of the soup in a food processor or blender (or use an immersion blender) and return to soup.

Stir in cream and simmer for 5 minutes. Taste and add salt and pepper as needed.