Heat oven to 400 and toss cut cauliflower with olive oil, salt and pepper. You can also add the sliced onion here and roast it along with the cauliflower, or you can add it raw when pureeing.
Roast until well browned. This gives a lot of the flavor so really let it get browned up but not burnt. This takes about 45 minutes depending on your oven.
Add to a blender with the garlic, onion, onion powder, garlic powder, stock or water and milk. Blend until mostly smooth, adding enough stock or water to make a thick soup.
Pour into a pot on the stove over medium and add the dried thyme, salt and pepper to taste. Simmer 10 minutes then serve.