Heat butter in a pot over medium heat.
Add chopped onion, sauté until translucent.
Stir in minced garlic, cook for 1 minute.
Add sliced mushrooms, cook until golden brown.
Sprinkle salt, pepper, and thyme leaves. Stir well.
Pour in chicken/vegetable broth, simmer for 15-20 mins.
Reduce heat, add heavy cream, simmer for 5 mins.
Use an immersion blender to blend soup until smooth.
Taste and adjust seasoning if needed.