Heat a 6 quart soup pot or Dutch oven over medium heat. Add olive oil and leeks and sauté for about 3-5 minutes. Add garlic, mustard seeds, turmeric, cumin, sea salt, and black pepper and let cook. Once mustard seeds pop (about 2 minutes) add carrots and cauliflower. Cook for a few more minutes. Add vegetable broth to the pot, and bring to a simmer. Cover and cook on low for about 45 minutes, or until cauliflower is soft.
Puree using a hand immersion blender. If you don’t have a hand blender puree in a blender working in batches until smooth. Serve warm in bowls and top with fresh chives.