Bring a pan of salted water to a boil. Add the cauliflower and cook for 8-10 minutes until the cauliflower is tender. Drain and set aside.
Meanwhile, add the milk, cloves, bay leaves, and peppercorns in a small pan. Bring to the boil and reduce to a simmer. Cook for 5 minutes, stirring continuously.
In a separate pan, melt the butter slowly. Whisk in the flour and whisk until a smooth paste forms. Cook for 2-3 minutes, but do not allow the flour to brown.
Remove the onion and spices from the milk and discard.
Using a ladle, slowly whisk in the warm milk. Whisk in 1 ladle of milk to the roux until well combined and smooth. Cook and whisk for 10 minutes until the sauce reaches the desired thickness.
Preheat an overhead grill to hot.
Remove the sauce from the heat and stir in the cheese and mustard powder. Stir until the cheese has melted. Season to taste with salt and a pinch of nutmeg.
Place the cooked cauliflower in a heatproof casserole dish and cover with cheese sauce. Top with the reserved grated cheese.
Place the cauliflower cheese under the grill and grill for 3-4 minutes until the cheese is bubbling and the top is golden brown.
