Prepare the ingredients: Slice the onion and mince the garlic cloves. Separate the cauliflower florets from the stem and rinse them in running water. Make sure to use the cauliflower stem by cleaning the grooves and chopping it into small blocks.
Sauté the aromatics: Boil the cauliflower. Preheat a pan on medium-low heat, and then add olive oil, onion, and garlic. Sauté the aromatics for four minutes and stir occasionally. While cooking the aromatics, prepare to boil the cauliflower. Add cauliflower to a pot and add water. Boil the cauliflower for minutes on medium-high heat with the lid closed. Drain the water once it is done boiling.
Mix béchamel sauce and cauliflower: Prepare a medium-sized bowl and pour the béchamel sauce into it. Add the aromatics, ground nutmeg, and celery salt, and stir to mix. Prepare a baking tray and evenly lay out the cauliflowers. The tray must be deep enough to hold the sauce. Pour the béchamel sauce mixture into the tray of cauliflowers.
Bake the cauliflower: Preheat the oven to 180–200°C (350–400°F) and bake for 35 minutes with the fan on. Thereafter, add the cheese and bake for another 10 minutes until the top begins to turn golden brown in colour, check the cauliflower every ten minutes to prevent burning.
Rest before serving: The cauliflower gratin should be rested for ten minutes before serving, which allows the sauce to thicken and intensify the flavors. Garnish with chopped parsley to serve.