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Bring a pan of lightly salted water to the boil and cook the cauliflower for 10 minutes. Add the kale to the top of the pan and cook for a further 2 minutes, then drain well and set aside.
Preheat the oven to 220°C/ 200°C Fan/Gas Mark 7.
Meanwhile, melt the butter in a pan, then add the flour and cook over a low heat for 1 minute, stirring continually. Add the mustard powder, stir and remove from the heat. Gradually add the milk a little at a time, stirring well to remove any lumps.
Return to the heat and bring to a simmer, stirring all the time. Once bubbling, cook for 2-3 minutes until thickened.
Stir in two-thirds of the Cheddar, half of the manchego and half of the vegetarian hard cheese and beat well. Add the mustard and ½ tsp ground black pepper and beat again. Add the drained cauliflower and kale to the sauce and toss well to coat. Carefully transfer to a 1-litre gratin dish.
Mix the remainder of the cheeses together and scatter over the top. Bake for 20 minutes until golden and cooked through. Serve with roast chicken or gammon as a delicious side dish.