1
Preheat your oven to 400°F (200°C).
2
Prepare the cauliflower by turning it upside down and carefully slicing around the tough stem using a sharp knife. Remove any leftover leaves, but do not separate the florets.
3
Place the whole cauliflower, stem side down, in a large pot with one inch of water and 1/4 teaspoon of salt. Cover tightly and bring to a rapid boil. Steam the cauliflower until you can pierce a floret with a knife, around 8 minutes.
4
Once cooked, lift the cauliflower from the pot and transfer it to your gratin dish to cool slightly. After a few minutes, turn the cauliflower upside down and cut around the core to break it into florets, arranging them in a single layer with the rounded tops facing up. Chop up the cooked core and scatter it among the florets. Set aside.
5
In a saucepan, combine 2 cups of milk, 1/2 cup of the reserved cooking water, and 1/2 cup of cream. Heat the mixture slowly over medium-low heat, ensuring it does not boil.
6
In a large saucepan, melt the 4 tablespoons of butter over medium heat. Stir in the flour to create a roux, cooking it for about 2 minutes while continuously stirring until it turns a light beige color.
7
Remove the roux from heat and slowly whisk in the heated milk mixture, ensuring a smooth blend. Return to the heat and bring to a gentle simmer, stirring constantly, until the sauce thickens slightly, about 3 minutes.
8
Season the sauce with 1/2 teaspoon salt, 1/8 teaspoon black pepper, and a pinch of freshly ground nutmeg. Adjust the seasoning as needed, and if the sauce is too thick, loosen it with more milk or cooking liquid with a whisk.
9
Once seasoned, pour the cheese sauce over the arranged cauliflower florets in the gratin dish. Lightly season the florets with an additional 1/8 teaspoon of salt and pepper, then sprinkle 1/4 cup of the grated Gruyère cheese on top, followed by the remaining cheese.
10
Place the gratin dish on a cookie sheet to catch any drips and bake in the preheated oven for about 25 minutes or until the sauce is bubbling and the cheese has turned a golden brown.
11
For make-ahead convenience, you can assemble the gratin a few hours prior and store it in the refrigerator. When ready to bake, preheat the oven to 375°F (190°C) and allow an extra 30 to 40 minutes for reheating and browning the cheese.