Melt the butter in a large pot over medium heat. Add the onion and cook until it softens.
Stir in the garlic and cook for one minute until fragrant.
Add the sliced mushrooms and thyme leaves. Cook until the mushrooms release their liquid and start to brown.
Pour in the vegetable broth. Bring the mixture to a simmer and cook for 15 minutes.
Remove the pot from the heat. Use an immersion blender to partially puree the soup, leaving some mushroom pieces for texture.
Return the pot to low heat. Stir in the heavy cream and season with salt and black pepper.
Heat the soup gently for a few minutes without letting it boil. Ladle into bowls and top each with a fresh thyme sprig.