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.
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