In a large pot over medium-high heat, soften the onion, celery and garlic in the butter. Add the remaining ingredients. Bring to a boil, then simmer for 15 minutes or until the vegetables are tender. Season with salt and pepper.
In a blender, purée the soup until smooth. Adjust the seasoning.
Meanwhile, with the rack in the middle position, preheat the oven to 400°F (200°C). Line a baking sheet with parchment paper.
Spread the Brussels sprout leaves on the prepared sheet. Drizzle them with the oil and season with salt and pepper. Roast for 8 minutes or until they begin to brown.
When ready to serve, ladle the soup into shooter glasses and garnish with the Brussels sprout leaves and pomegranate seeds. Season with pepper and drizzle with olive oil, if desired.