Bring a pot of water to a boil for the macaroni. Salt it liberally and cook pasta to just shy of al dente—about 7 minutes for whole wheat, about 6 for white pasta.
 
Place stock, garlic and onion in a Dutch oven and bring to a boil. Add whole head of cauliflower, cover and simmer 15-20 minutes until the cauliflower is very tender. Remove to a plate and strain the stock.

 
In a deep skillet or large saucepot, melt butter over medium to medium-high heat. Whisk in flour, cook for a minute then stream in stock. Bring to a boil, add half-and-half and let sauce thicken; stir in cheddar cheese. Season sauce with mustard, nutmeg, salt and pepper.
 
Preheat broiler to high and place a rack in the center.
 
Separate cauliflower into small florets and toss with sauce and pasta,. Place in a baking dish and top with Parm. Place in oven and broil to brown, about 3-5 minutes. Garnish with chives and parsley, and serve.