Bring water to a boil for pasta. Salt the water and undercook the pasta by 2 minutes, about 5 to 6 minutes.
While the water comes to a boil, in a covered Dutch oven heat extra-virgin olive oil over medium to medium-high heat. Cut the core away from the cauliflower and set the head into the pot and add 1 cup stock, cover and steam 12 to 15 minutes until tender. Remove and separate the cauliflower into florets. Discard water from pot.
Over medium heat in the same pot, melt the butter, then add onions and garlic. Saute for 3 to 4 minutes, until tender. Add in the flour and whisk for 1 minute. Whisk in the whole milk and season with salt, pepper, nutmeg, and sage. When thickened, stir in the Dijon mustard and 2/3 of the combined cheeses.
Drain the pasta, add to the cauliflower, stir in the cheese sauce, transfer to baking dish, top with remaining cheese. Cool completely and cover for a make-ahead meal.
Preheat the oven to 400 degrees F.
Bake until brown on a baking sheet to catch the bubble-over. The casserole will take 40 to 45 minutes to heat through.
Garnish bowls of cauli-mac-n-cheese with chopped spicy watercress.