Clean mushrooms, trim stems, then quarter.
Heat a large frying pan over medium heat. When it is hot, add the olive oil and butter. Stir until the butter melts.
Add the mushrooms and onions to the frying pan and stir to coat. Lower the heat if necessary. The mushrooms will give up a lot of water in the initial cooking process. After the water has reduced, lower the heat to low and cook the mushrooms and onions until they begin to take on a caramelized appearance. Watch carefully and stir as needed so that they do not burn.
Once the mushrooms and onions have caramelized, add the garlic. Continue to cook until garlic is fragrant, 1-2 minutes. Add two cups of the chicken broth and the thyme to the pan with the mushroom mixture. Bring to a simmer over medium-low heat and allow the liquid to reduce by half. Take pan off the heat and allow everything to cool somewhat.
Transfer the mushroom and broth mixture from the pan into a blender. (Note: if you would like some chunky mushroom pieces in your soup, reserve a small amount to add back into the soup in the next step.) Pulse-blend a few times to begin to incorporate the ingredients, then turn the blender on high and process until fairly smooth. With the blender running, pour 1 cup of cream into the ingredients in the blender and process for a few seconds until well blended.
Pour contents from the blender into a saucepot. Stir in the remaining 1 cup cream, the remaining 1½ cups chicken broth or stock, Worcestershire sauce, reserved mushrooms (if using), and seasoning salt.
Cook and stir over medium heat until soup is hot, but not boiling. Taste for flavor; adjust seasonings as needed. If you prefer a thinner soup, add a little more chicken broth.