Heat a large pot over medium heat and melt the butter.

Add the diced onion and minced garlic. Cook, stirring occasionally, until soft and fragrant, about 4 to 5 minutes.

Add the sliced shiitake and crimini mushrooms. Stir and cook until the mushrooms are nicely browned and most of their liquid has evaporated, about 8 to 10 minutes.

Pour in a small splash of broth and scrape the bottom of the pot with a wooden spoon to loosen any browned bits. Let the broth reduce for about a minute.

Stir in the vegetable broth and raise the heat until the soup reaches a gentle simmer.

Add the fresh thyme leaves and season with salt and black pepper. Let the soup simmer for 10 minutes to meld the flavors.

Turn the heat to low and stir in the heavy cream until fully incorporated. Keep the soup below a boil to prevent curdling.

Remove the pot from the heat. Use an immersion blender to puree the soup until silky smooth, or let the soup cool slightly and blend in batches in a countertop blender.

Taste and adjust seasoning as needed. Serve warm.