To prepare the white sauce, firstly melt the butter and add the flour, stirring continuously with a wooden spoon until the flour has been fully incorporated in the melted butter.
Gradually add the milk, a little at a time, continuously stirring until a smooth sauce has been achieved.
Add the cheese and the egg yolk at this stage to give a richer sauce.
Cover with a layer of clingfilm to prevent a skin forming.
Preheat the oven 180°C/350°F/Gas Mark 4.
Grease a large baking tray using half the butter.
Place the cauliflower florets and garlic into the baking tray and drizzle with olive oil, then add the remaining butter.
Cover with foil and place in the preheated oven for 30 minutes.
Remove the foil and pour over the Mornay sauce.
Sprinkle over the Parmesan cheese.
Return, uncovered, to the oven for a further 8-10 minutes until golden.