In a large pot over high medium-heat, warm 1 tablespoon oil and butter. Once the butter is melted and hot, add the mushrooms and spread out into an even layer. Let cook for 5 minutes, undisturbed, until deeply browned. Transfer mushrooms to a plate and set aside.
Add the remaining 1 tablespoon oil to the pot along with the onion, carrot, celery, and ½ teaspoon salt. Cook, stirring occasionally, for about 3-5 minutes, or until softened. Stir in the garlic, thyme, and browned mushrooms and cook for about 30 more seconds.
Stir in the rice, water, coconut milk, soy sauce or tamari, and remaining 1 teaspoon salt. Increase the heat to high, cover, and bring to a boil. Once boiling, reduce the heat to medium-low to maintain a rapid simmer for about 40-50 minutes, or until the rice is tender, stirring halfway through. If the water level looks low just add more (this will depend on the rice you use).
To thicken the soup up a bit (optional), remove about 1 cup of the soup and add it to a high-speed blender. Blend it up until smooth. Stir back into the pot along with the lemon juice. Taste and add more lemon or salt, if needed. If the soup is too thick, just stir in a bit of water to thin it out. Taste and add more salt or lemon, if needed.
Serve topped with black pepper and fresh herbs, if using.