In stockpot over medium heat, warm 2 Tbsp oil. Add onion, celery, and carrot; sauté until onion is soft and translucent, approximately 6 minutes. Add garlic, turmeric, and black pepper to pot; sauté for 1 minute. Stir in stock and flavor sachet and bring mixture to a simmer. Cover stockpot, reduce heat to low, and simmer gently for 10 minutes.