BLEND: Dice the onion, peel the garlic cloves and roughly chop the cauliflower florets, then add them to a large dutch oven, along with the chicken broth and dried thyme. Over medium heat, bring the broth to almost a boil, then reduce the heat to an even simmer. Cook until the cauliflower is just fork tender, about 15 minutes. While the cauliflower is softening, peel and slice the carrots. Carefully transfer the dutch oven off the heat and use an immersion blender to puree the soup together until smooth.
SIMMER: Return the blended soup back to the stove top. Add the sliced carrots and bring back to a simmer. Peel and dice the sweet potato into bite sized pieces. The carrots will need longer to soften than the sweet potato, so simmer the carrots in the soup for 15 minutes before adding the diced sweet potato and simmering until just tender, about another 10 – 15 minutes. Add the roast chicken and salt, to taste and cook until the chicken is completely warmed through before serving.