Preheat the oven to 375°F. Place the cauliflower in a lightly greased 9×9 baking dish or large cast-iron skillet, and then toss the florets with the olive oil and 1/2 teaspoon salt and 1/2 teaspoon pepper. Bake for 15 minutes then remove from the oven, leaving the cauliflower in the dish.
Meanwhile, bring a large pot of water to a boil, add about 1 tablespoon of salt and the pasta, and then boil until the pasta is al dente, or softened but still a little chewy.
Once the pasta is done, drain and rinse the pasta and then pour into to the dish with the cauliflower. Add the shredded Gruyere and then stir to combine the cauliflower, pasta, and Gruyere.
Whisk together the cream, garlic, mustard, nutmeg, cayenne, 1/2 teaspoon kosher salt, and 1/2 teaspoon pepper. Pour over the pasta and stir until well combined. Evenly sprinkle over the pasta the Parmesan cheese and then bake uncovered for 45-50 minutes or until brown and bubbling. Allow to cool for 10 minutes so the cream can set and then serve warm.