A combination of two great classics, and outrageously indulgent side dish for serving alongside steak or roast beef - though makes a fine main course in its own right if accompanied by a sharply-dressed green salad and a few tomatoes.
This dish is a bit of an oven-hogger, so either serve with something you can cook on the hob, or make the day before and simply return to the oven at 170°C for 30 minutes while your roast rests. If you do this, add the remaining cheese when you reheat rather than when you bake.
1. Heat the oven to 160°C.
2. Slice the potatoes into as fine-a-discs as you can manage - a mandolin is great if you have one, if not, a very sharp knife and some patience will do the trick nicely.
3. Mix the potatoes, sliced shallot, cream and milk in a large bowl, and use your hands to ensure everything is well combined. Grease a medium-sized baking dish with a dot of butter, then layer the potatoes and shallots, seasoning with a little salt and pepper between each layer. Pour the liquid over the potatoes so that they are just covered. Cover with foil and bake for 75 to 90 minutes, removing the foil after 45, until the potatoes are just cooked through - you can test with a small sharp knife.
4. While the potatoes are baking, cut the cauliflower into small florets (around 3cm wide), discarding most of the stalk. Bring a pan of salted water to the boil, add the cauliflower and blanch for 2 minutes. Drain immediately.
5. Next, make a bechamel. Take a small saucepan and warm over a low heat. Add the butter, and once melted, add the plain flour and stir vigorously for 2-3 minutes, while the flour cooks (it should start to smell slightly biscuity). Add a small amount of milk, and stir to bring back to a smooth paste. Continue with the rest of the milk, adding a small amount at a time, until it is all combined and you have a thick sauce. Remove from the heat and add the mustard, nutmeg and three-quarters of the cheese and season to taste with salt. Combine the bechamel, drained cauliflower and set to one side until the potatoes are cooked.
6. When the potatoes are cooked, remove from the oven and turn the temperature up to 200°C. Top with the cauliflower mix and the remaining cheese and bake for 25 minutes, until golden, bubbling and delicious. Allow the bake to stand for 10 minutes until serving.
Serves 6 as a side
For the gratin
1 banana shallot, very finely sliced
250ml double cream
200ml whole milk
Butter for greasing
Salt and Pepper
For the cauli-cheese topping
1 small head cauliflower
50g plain flour
250ml whole milk
2 tsp dijon mustard
A few grates of nutmeg
150g mature cheese