I have made Mushroom soup a gazillion times and have tried and tried to get that wonderful deep mushroom flavor without much luck. I have finally found it! Larry says I should can it and sell it. And Cream of Mushroom soup is his favorite. :) Pretty high praise in my books.
In a 12 inch heavy skillet on medium high heat, melt butter and add olive oil. When the butter quits sizzling, put in all of the sliced mushrooms at once and stir to coat them with butter/oil mixture. Keep stirring occasionally as the mushrooms release their liquids. Add soy sauce and stir.
When the liquids are almost gone, turn down the heat to medium and let the mushrooms brown in the skillet stirring frequently until nice and golden. There should be a dark brown (Not burnt black) coating in the bottom of the skillet by the time they are browned.
Add 1/4 c of sherry and scrape the brown bits off the bottom of the pan (deglaze the pan). Let that sherry evaporate completely while stirring the mushrooms. Add another 1/4 c of sherry and stir to deglaze again. There should still be a little dark liquid in the pan.
Add all the chicken broth, thyme and mushroom base. Let simmer for 15 to 20 minutes. Remove from heat and reserve. (I made this base and then put it into a container in the fridge to finish later. You could freeze this "as is" in zipper bags and make your soup at your convenience.)
In a Large saucepan, melt butter and add flour to make a roux. Cook roux for 1-2 minutes to avoid "raw" flour taste. Whisk in whole milk and bring to a simmer, whisking and stirring to avoid scorching. When very thick, add the heavy cream and seasonings. (Remember that the Mushroom Mixture may be slightly salty from the soy and mushroom bouillion base so do not oversalt this cream base.)
Whisk in the Mushroom and chicken broth mixture and heat to just below boiling. Turn burner to very low and let cook/simmer for 15 to 30 minutes, stir occasionally.
Ladle into serving bowls and top with your choice of garnish.