Heat olive oil in a large pot over medium heat. Add onion and cook until softened, about 5 minutes.

Add garlic and cook for another minute until fragrant.

Add mushrooms and cook until browned and softened, about 10-15 minutes.

Pour in vegetable broth and sherry (if using). Bring to a boil, then reduce heat and simmer for 15 minutes.

Carefully blend the soup using an immersion blender or transfer to a regular blender. Blend until smooth.

Stir in fresh dill, salt, and pepper to taste.

Serve hot.