A cosy, warming dish that would be a perfect side with a baked ham or some white fish. It also makes a very satisfying main meal just in itself, though you won’t win many points toward your 5-a-day. (Not that this would stop me.)
Slice the potatoes either by hand or using a processor with a slicing attachment on a medium setting. You don’t want wafer-thin potato slices, but neither do you want thick chunks.
Feeds 4 as a main, up to 8 as a side
Butter, for greasing
1 kg red potatoes, scrubbed and unpeeled, thinly sliced
2 – 3 leeks, halved lengthways and cleaned, then thinly sliced
4 rashers of smoked streaky bacon, finely chopped
Salt and pepper
250g grated cheese (I use a mix of cheddar, mozzarella and parmesan, but any good melters will work)
1 tsp cornstarch / corn flour
1 tsp grainy mustard
1 tsp dijon mustard
Preheat the oven to 400F / 200C / 190C Fan.
Butter a baking dish. Set aside.
Mix the cheeses together in a bowl along with the cornstarch.
To assemble the gratin, start with a layer of slightly overlapping potatoes. Season the potatoes with salt and pepper, then sprinkle over 1/3 of the chopped leeks, 1 of the chopped rashers of bacon and 1/3 of the cheese mixture. Repeat until you have at least three layers, ending with a cheese layer.
When you’ve finished layering up, sprinkle over the final chopped rasher of bacon on top.
Mix the mustards and milk together, then slowly pour over the potatoes. Cover with foil and bake for 1hour 15 minutes, then remove the foil and bake for a further 10 – 15 minutes, or until bacon has crisped, cheese has melted and a knife slides easily into the potatoes. (No one likes an al dente potato gratin.)
Let stand for a couple minutes, then slice and serve.
Make ahead: Gratin can be assembled up to a day in advance. Remove from the fridge at least an hour before baking.