In a large pot, over medium-high heat, melt 2 tablespoons butter. Add in mushrooms and onions. Saute until tender, about 5 minutes. Add garlic and cook additional minute, until garlic is fragrant.
Add cream, broth and thyme to mushroom mixture. Stir. Bring to a boil over medium-high heat, then reduce heat to simmer for 10 minutes, stirring occasionally.
Use an immersion blender* to blend soup until smooth and creamy. Serve hot.
Notes
*Alternate method - working in batches, transfer soup to stand blender and pulse until smooth.