Sauté the Aromatics. In a large pot, melt butter over medium heat. Add the onion and garlic, and sauté until soft and fragrant.
Cook the Mushrooms. Add sliced mushrooms to the pot and cook until they release their moisture and become golden brown. This step intensifies the earthy flavor.
Thicken the Base. Sprinkle the flour over the mushrooms and stir to coat. Cook for 1–2 minutes to remove the raw flour taste.
Add the Broth. Gradually pour in the chicken or vegetable broth, stirring constantly to avoid lumps. Bring to a gentle boil, then reduce heat and simmer for 10 minutes.
Blend for Creaminess. Use an immersion blender to puree the soup until smooth. To retain some of the texture, try removing half of the soup to puree and add it back to the unblended portion.
Stir in Cream. Add heavy cream or milk and heat gently. Season with salt, black pepper, and fresh herbs if using.
Serve. Ladle the soup into bowls and garnish with fresh parsley or thyme. Pair with crusty bread or garlic knots for a comforting meal.