Preheat oven to 425º
clean and chop the cauliflower and the onion, you don't need to be precise here as long as the pieces are roughly the same size
Toss cauliflower and onion with 1 ½ tablespoons of the oil
Spread evenly on a baking sheet, and sprinkle with salt and pepper
Slice top of garlic head off to expose the cloves inside, wrap in a double layer of aluminum foil, and drizzle with remaining oil, and fold the foil over the top
Place on the baking sheet with the cauliflower and onion, then place the baking sheet in the oven
roast for 12 minutes, then flip vegetables over, and bake until tender, another 12-15 minutes
Scoop tender cauliflower and onion into a large blender or food processor (depending on the size of your blender, you may need to do this in batches
Scoop garlic cloves out of paper skins and add them to the blender as well
Add ½ - 1 cup of the chicken broth
puree or blend until smooth
Transfer to a pot
add remaining chicken broth, cheese, mustard, and lemon juice
Heat, stirring until cheese in incorporated
When soup is hot, serve with croutons and parsley (if desired)