Heat the butter in a large soup pot or dutch oven pot over medium high heat. Add the sliced mushrooms and chopped onion, sauté for about 5 minutes, or until the onion is translucent and the mushrooms have softened and released liquids and is starting to brown.
Add the minced garlic, thyme, rosemary, salt and black pepper. (Optional stir in 1 tablespoon of all purpose flour for a thicker creamier soup). Stir well and sauté for another minute or until fragrant. (Optional to reserve a few mushrooms for topping)
Stir in the potatoes and vegetable broth (start with 4 cups and add more if needed). Bring to a boil while stirring often, then reduce the heat and let the soup simmer for about 20 minutes or until the potatoes are soft and fork tender.
Stir in the heavy cream and take out ⅓ - ½ of the soup and add to a high speed blender. Blend until smooth and return to the pot and stir well. Taste the soup and add more salt and pepper if needed.
Serve the soup hot topped with a drizzle of cream, optional reserved fried mushrooms and some fresh thyme or rosemary.
