In a 4-qt. saucepan, heat butter over medium heat. Add mushrooms and saute for about 5 minutes. Remove a few mushrooms from the pan and reserve for garnish. Add onions, carrots, celery, and garlic. Season with salt and cook, stirring occasionally until tender and lightly browned, about 10 minutes.
Add the chestnuts and cook for 4 minutes. Pour in the chicken stock and add the thyme, and rosemary; bring to a boil then reduce the heat. Partially cover the pan and simmer over low heat for about 30 minutes.
Once cooked, puree soup with an immersion blender or in batches in a regular blender. Whisk in creme fraiche and black pepper. Season with more salt to taste. Serve hot and garnish with reserved mushrooms and fresh snipped chives.
Notes
Storage: Store leftovers in an airtight container in the refrigerator for 3–4 days.
Freezing: Freezes well for up to 2 months; thaw overnight in the fridge and reheat gently.
Chestnut Tip: Pre-cooked chestnuts (vacuum-packed, jarred, or canned) are a quick and reliable option that saves peeling time.