Melt the butter and olive oil in a large saucepan over medium heat. Sauté the mushrooms. After 8 to 10 minutes of cooking, add the sliced mushrooms and simmer, stirring regularly, until the mushrooms start to brown and release their juices.
Flavor with Aromatics: Saute the chopped onion for 5 minutes, or until it softens. Then, after another minute or two, add the thyme and minced garlic and continue cooking until fragrant. Add salt and pepper to taste.
Alternately, you may make a Roux: While the mushrooms are cooking, whisk in the flour and simmer for another minute or two. The soup will thicken as a result of this.
Pour in the broth and simmer, stirring occasionally, until combined. After the ingredients come to a boil, lower the heat and simmer for 10 to 15 minutes to let the flavors combine.
If you want your soup smoother, you may mix it with an immersion blender or gently transfer parts of it to a blender.
Toss in the heavy cream or half-and-half and stir while gently heating, being careful not to boil. Add more or less salt and pepper as desired.
Before serving, spoon the soup into individual bowls and top with chopped fresh parsley, if preferred.
On cold nights, slurp up some of this creamy mushroom soup.
