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1
Bring a large pot of water to a rapid boil. Salt it heavily and add a squeeze of lemon juice. Separate the cauliflower into even-sized florets — uniformity ensures even cooking. Blanch for 5–7 minutes until a knife point meets slight resistance; the florets must not be fully tender, as they will continue to cook under the sauce. Drain thoroughly in a colander, shaking away all surface water so none dilutes the Mornay.
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2
Make a medium béchamel: melt 40 g butter in a heavy saucepan over moderate heat, add 40 g flour, and stir to a smooth roux. Cook the roux briefly without colouring, then add the 500 ml milk in a steady stream, whisking constantly. Season with salt, white pepper, and nutmeg. Cook, stirring, until the sauce is thick — thicker than a standard béchamel but still pourable, coating the back of a spoon with a firm, even layer that does not run when a finger is drawn through it.
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3
Remove the pan from the heat. Whisk in the 2 egg yolks one at a time, then add the 100 g of finely grated Gruyère (or Gruyère and Parmesan mixture), stirring until fully melted and incorporated. This is the point on which the dish lives or dies: the sauce must be thick enough to coat and cling to every floret without sliding to the bottom of the dish — if it runs freely, it will pool and leave the cauliflower bare; correct by returning briefly to low heat and stirring until it tightens.
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4
Butter a gratin dish generously. Arrange the drained florets in a single, even layer. Nappé the Mornay over them generously, ensuring every floret is well coated. Scatter the additional 50 g of grated Gruyère evenly over the surface and dot with the small pieces of butter.
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5
Transfer to an oven preheated to 220 °C, or place under a hot grill. Gratinée for 15–18 minutes in the oven, or 8–10 minutes under the grill. The dish is ready when the sauce bubbles fiercely across the entire surface, the crust is deep golden-brown with dark, almost charred spots, and the edges are crisp and caramelised. Serve immediately in the gratin dish.