Preheat the oven to 425°F with a rack in the center position
Pat the chickpeas dry with a paper towel
Combine the chickpeas, cauliflower and carrots to a rimmed sheet pan
In a small bowl, combine 1 tablespoon of the curry powder, ½ teaspoon of the turmeric, ½ teaspoon of the salt, and ⅛ teaspoon of the cayenne (if using)
Drizzle the vegetables with 3 tablespoons of the olive oil and sprinkle with the spice mixture; toss until coated
Roast until the vegetables are tender, about 20 minutes
Meanwhile, heat the remaining 1 tablespoon of olive oil in a large stockpot
Once the oil is glistening, add the onions and cook stirring until beginning to soften, 4 minutes
Add the ginger and garlic and cook, stirring until fragrant, 1 more minute
Add the remaining 2 tablespoons curry powder, ½ teaspoon turmeric, ½ teaspoon sea salt, and ⅛ teaspoon cayenne (if using) and stir until evenly coated
Stir in the stock, coconut milk, sugar, and lemon juice, until fully combined and heated through
Add half of the roasted veggies and chickpeas to the soup
Using an immersion blender, blend the soup in the pot until very smooth
(Alternatively, transfer the soup to the base of a blender, let cool briefly, and blend until smooth.) Stir the remaining vegetables and chickpeas into the soup
Divide the soup between 8 bowls and serve topped with cilantro