If making this to serve on Cook Day, preheat the oven to 400°F. If making this for later in the week, you'll preheat the oven on the night of.

Fill a soup pot or Dutch oven with water and bring to a boil.

Salt the water and cook the pasta until 2 minutes shy of al dente.

Drain. Dry the pot and return it to the stove.

In the same pot, heat the EVOO (1 turn of the pan) over medium to medium-high heat. Cut the core out of the cauliflower (the head stays intact) and set the head in the pot. Add the stock, cover, and steam until tender, 12 to 15 minutes. Transfer to a plate. Discard the cooking liquid and wipe out the pot. When cool enough to handle, separate the cauliflower into florets.

Meanwhile, return the pot to medium heat and melt the butter.

Add the onion and garlic and cook to soften, 3 to 4 minutes.

Sprinkle with the flour and stir for 1 minute. Whisk in the milk and add the sage; season with salt, pepper, and a few grates of nutmeg. Cook until thickened. In a bowl, combine all 3 cheeses. Stir the mustard into the sauce, then stir in two-thirds of the combined cheeses until melted.

Add the pasta and cauliflower to the sauce. Transfer to a deep 9-by-13-inch baking dish and top with the remaining cheese.

Make-ahead: Let cool and refrigerate. [Night of: Return the casserole to room temp while you preheat the oven to 400°F.]

Place the casserole on a baking sheet and bake until heated through and browned, 40 to 45 minutes.

Serve in bowls topped with chopped spicy watercress.