In a medium sauce pot, add the mushrooms, garlic, thyme and a couple drops of water to prevent sticking. Sauté until mushrooms are softened and reduced, approx 5 minutes.
Add the broth, miso paste and lemon juice and bring to a boil, then reduce heat and simmer for about 5 minutes. Turn off heat, stir in the milk, and then using an immersion blender, puree your soup until creamy. Add salt and pepper to taste and serve with crusty bread.
If you don't have an immersion blender, carefully transfer the soup to a blender in portions and process until smooth. Use extreme caution when blending hot liquids.