Heat oil in a soup pot over medium heat. Add onions and mushrooms and cook until tender, about 5-6 minutes. Mix in the flour until well combined. Slowly mix in stock. Stir in the sage. Bring to a gentle simmer and cook 15 minutes. Stir in the light cream or milk and season with salt and pepper. Divide among 4 soup bowls. Drizzle with sherry, garnish with sage leaves, and serve.