Preheat the oven to 425 degrees F.
Place the cauliflower, garlic, and onion on a baking sheet; drizzle with two tablespoons of oil and sprinkle the thyme and cayenne on top.
Roast until lightly browned, about 20 to 25 minutes. If the garlic begins to brown too quickly removed it from the pan sooner.
Toss veggies halfway through cooking.
While the cauliflower is roasting, place the broth in a blender or food processor. Add the kale, leftover oil, lemon, salt, and yogurt if desired. Blend well.
Remove the veggies from the oven. Carefully add them to the kale broth and purée until smooth and consistent. Add a little water if it is too thick.
Ladle the soup into bowls and garnish with kale and cauliflower if desired.