1. In a large Dutch oven or pot, heat oil over medium heat. Add onion. Season with salt; cook, stirring occasionally, until softened, 5 to 7 minutes.

2. Add mushrooms, potato, broth, and enough water (4 to 5 cups) to cover. Bring to a boil; reduce heat to medium, and simmer until mushrooms and potato are tender, about 20 minutes.

3. Using a blender or an immersion blender, puree broth and vegetables until smooth. If using a blender, work in batches and fill only halfway, allowing heat to escape: Remove cap from hole in lid, cover lid firmly with a dish towel, and blend. Transfer to a clean pot as you work. Adjust soups consistency with a little water if necessary. Season with salt, pepper, and lemon juice.