Heat oil in a large soup pot over medium-high heat. Saute onion until translucent, about 2-3 minutes. Add garlic and mushrooms and reduce heat to medium. Cook for 5 minutes, stirring occasionally.

Add broth and oats and bring to a boil. Reduce heat and simmer 8 minutes. Remove soup from heat and allow to cool slightly. Stir to release heat. Once the soup is warm, but not boiling, add the miso and pepper. Use an immersion blender to puree until smooth. A regular blender may be used in place of immersion blender.

Garnish with parsley and serve.