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Preheat the oven to 375°F (190°C) and grease a baking dish with butter.
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Roast the cauliflower – Spread cauliflower florets on a baking sheet, drizzle with olive oil, season with salt and pepper, and roast for 25 minutes until tender and lightly browned.
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Prepare the cheese sauce – In a saucepan, melt butter over medium heat, whisk in flour, and cook for 2 minutes to form a roux.
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Add the milk – Gradually whisk in warm milk and cook until thickened, about 5 minutes.
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Stir in the cheeses – Remove from heat and mix in Gruyère, cheddar, nutmeg, salt, and pepper until smooth and melted.
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Combine everything – Add the roasted cauliflower to the cheese sauce and stir until evenly coated.
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Transfer to a baking dish – Pour the mixture into the prepared dish and spread it evenly.
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Add the topping – Mix breadcrumbs with Parmesan, sprinkle over the top, and drizzle with olive oil.
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Bake until golden – Place in the oven for 25–30 minutes until the top is golden and bubbling.
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Let it rest – Cool slightly before serving to allow the cheese to set.