In a large pot, heat the olive oil or butter over medium heat. Add the onion and garlic and sauté for 3-4 minutes until softened and fragrant.
Add the sliced mushrooms to the pot and cook for another 5-7 minutes until they start to release their juices and brown.
Add the vegetable or chicken broth to the pot, along with the dried thyme. Bring the soup to a boil, then reduce the heat and let it simmer for 15-20 minutes.
Remove the pot from the heat and let it cool slightly. Using an immersion blender or a regular blender, puree the soup until smooth.
Stir in the lemon juice and season the soup with salt and pepper to taste.
Serve the Mushroom Soup Without Cream hot, garnished with fresh herbs or croutons.