Prepare gravy according to package directions, using vegetable stock in place of water. Set aside and keep warm.
Sauté mushrooms in butter in large skillet over high heat until brown and tender. Add to gravy mixture.
In same pan, sauté onions until just tender. Add vegetable oil if necessary. Add to gravy mixture.
Deglaze pan with sherry. Add liquid to gravy mixture
Use an immersion blender to purée the soup
Simmer for 15 minutes, stirring frequently.
Add cream, thyme, and red pepper flakes, and simmer for an additional 5 – 10 minutes, stirring frequently.
Optional: Serve garnished with chopped cooked bacon, pancetta, or prosciutto.