- 25g butter
- 1 tsp olive oil
- 1 medium onion, halved and finely sliced
- 1 clove garlic, crushed
- 900g Maris Piper potatoes (or King Edward potatoes), peeled weight
- 1 level tsp cornflour
- 1 level tsp Dijon mustard
- 800ml semi-skimmed milk
- 100ml reduced-fat crème fraîche
- 50g reduced-fat mature Cheddar, grated
Pre-heat the oven to 190C/170C Fan/Gas 5. Use 10g of the butter to grease a large ovenproof dish. Heat the rest of the butter with the oil in a large pan, then cook the onion over a low heat until soft. Add the garlic and cook for another minute.
Slice the potatoes evenly to a thickness of about a £1 coin. Add to the pan.
Mix the cornflour and mustard with a splash of milk until smooth, then mix with the rest of the milk. Add to the pan with the crème fraîche and heat until simmering – stir very gently a few times, to prevent the potato slices sticking together. Simmer for about 6 minutes, until the potatoes are just starting to soften.
Use a slotted spoon to lift out and drain the potatoes, then put in layers in the dish. Level the top.
Pour in enough of the milk mixture to cover all of the potatoes except for the top layer.
Sprinkle the cheese evenly over the top layer of potatoes. Bake for 45-60 minutes, or until the potatoes are soft but not mushy, and the top is golden brown and slightly crispy. Serve warm.