Preheat the oven to 200°C/fan 180°C/gas mark 6. Place the milk in a large saucepan with the onion, cloves, nutmeg and bay leaf and bring to a simmer. Remove from the heat and leave to infuse for 10 minutes, then strain and discard the onion, bay leaf and cloves.
Melt the butter in a large saucepan and add the flour. Cook over a medium heat for 2 minutes, then remove from the heat and gradually add the infused milk, stirring constantly to prevent lumps from forming. Place back over a medium heat and bring to the boil for 5 minutes, until thickened, then add the cheese, mustard and seasoning.
Add the cauliflower to a pan of boiling water and cook for 3-4 minutes, until just softened. Drain and mix with the cheese sauce. Place in a medium baking dish and top with the puff pastry, cutting off any excess pastry from around the edges. Brush well with the beaten egg and pierce a hole in the middle of the pastry to allow steam to get out.
Bake for 25-30 minutes, until the pastry is deep golden. Leave to sit for 10 minutes before serving with a green salad.