Place the oil in a large nonstick skillet and heat over medium-high heat. Add the onion and ginger and cook for 4 minutes. Stir in the coriander seeds, cumin seeds, turmeric, and salt and toast for 15 seconds and then add the tomatoes and sweet potatoes, and cook until they are tender, about 12 to 15 minutes. Add the cauliflower and stir well to combine. Place a tight fitting lid on the pan and cook until the cauliflower is crisp- tender, about 5 to 8 minutes.