In a large pot, heat the olive oil over medium heat. Add the diced onion and sauté for about 5 minutes, until translucent.
Stir in the minced garlic and sliced mushrooms. Cook for another 5-7 minutes, until the mushrooms are softened and browned.
Add the dried thyme, salt, and black pepper. Stir to combine and cook for 1 minute to release the flavors.
Pour in the vegetable broth and bring the mixture to a boil. Reduce heat and let it simmer for 15 minutes.
Add the chopped spinach and cook for an additional 5 minutes, until wilted.
Use an immersion blender to puree the soup until smooth. Alternatively, carefully transfer the soup in batches to a blender and blend until smooth.
Return the soup to the pot (if using a blender) and stir in the heavy cream and lemon juice. Heat through for about 5 minutes, but do not boil.
Serve hot, garnished with fresh parsley.