Heat 3 tablespoons of olive oil in a soup pot over medium-high heat until shimmering. Add the onions and mushrooms and cook until the onions are translucent. Add the broth and turn down the heat a bit. Allow soup to simmer gently for 20 minutes. Remove the soup and allow to cool for 5 minutes.
Working in batches, puree soup in the blender. Add salt and pepper to taste.
Before serving the soup, add a squeeze of fresh lemon to each bowl and drizzle with additional olive oil, if desired. Garnish with a slice of lemon.