100g medium-mature cheddar or gruyere, coarsely grated
250ml double cream
Heat oven to 200C/180C fan/gas 6. Heat the oil in a pan and fry the bacon for 5 mins until just starting to crisp. Add the onion and sizzle for 12 mins until soft and golden.
Meanwhile, tip the potatoes into a large pan of water, making sure they are well covered. Bring to a gentle boil, then add the cauliflower and cook everything for 6-7 mins until both are tender. Tip into a colander and drain thoroughly.
In a gratin dish, layer the potato and cauliflower, then scatter with bacon, onion and a small handful of cheese. Drizzle each layer with a little cream and season. Continue until all the layers are finished, pour over the remaining cream and scatter with the last of the cheese. Bake for 30 mins until bubbling. Remove from the oven and set aside for 1 min to rest. Serve straight from the dish with a crisp green salad.