Remove the green leaves from the cauliflower, remove the stem and chop the florets coarsly.
Wash the spinach well, dry in a towel or salad spinner and chop coarsely.
In a stock pot, heat the oil over medium heat and add the garlic, cook for just 30 seconds.
Add the cauliflower, broth and salt; bring to a boil. Reduce heat and simmer for 15 minutes. Add the spinach and cook another 5 minutes, until the spinach is wilted and the cauliflower tender. Mix in the mustard and thyme.
Using a hand-held immersion blender, puree the soup in the pot until smooth.