Heat oil in a three-quart saucepot over low heat. Add onions, shallots, celery, and salt. Cook vegetables until tender. Add mushrooms and increase heat to medium. Cook 10 minutes, stirring often. Add chicken broth, garlic and thyme. Bring back to a simmer and cook 10 more minutes.
In a separate skillet, make a light-blonde roux by melting butter and stirring in flour. Add to simmering broth mixture. Cook three to four minutes and add cream. Freezes well. Yield: two quarts