Preheat the oven to 400°F.
Season the leeks, cauliflower, and garlic with the olive oil, salt, and pepper.
Divide the vegetables between 2 baking sheets, and roast for 20 to 30 minutes, or until slightly browned, rotating the pans halfway through the cooking time.
Remove from the oven, allow the vegetables to cool slightly, and then transfer them to a blender.
Add 1½ cups of broth, and blend until smooth.
Add the remaining 1½ cups of broth in increments and blend until your desired consistency is reached. If you have an immersion blender, you can put everything right into a large saucepan and blend on the stove.
Transfer the soup to a large saucepan, stir in the thyme, and simmer over low heat for 10 minutes.
Add more chicken broth if the soup gets too thick.
Taste and add more salt and pepper if necessary, remove the thyme sprigs, and serve hot topped with the scallions.