Ingredients

• 500 g waxy potatoeswaxy, such as Charlotte; floury ones break up when parboiled
• 1 large onionthinly sliced
• 8 large eggs
• 120 g mature cheddara mature block gives stronger flavour for less cheese
• 30 g butter
• 1 tbsp olive oil
• 2 tbsp whole milk
• 1 tsp thyme leaves
• 1/2 tsp Dijon mustard
• 3/4 tsp fine salt and freshly ground black pepper

Method

1. 01

   Parboil the potatoes

   Boil the thinly sliced potatoes for 4 minutes. Just until the edges yield but the slices still hold together. Drain and leave to steam dry for 5 minutes; wet slices will not take on colour.

   500 g waxy potatoes

   As good at room temperatureAn hour later it is still excellent, which makes it genuinely useful for a crowd.

2. 02

   Caramelise the onion

   Cook the sliced onion in half the 30 g butter with a pinch of salt in an ovenproof frying pan over medium-low heat for 15-20 minutes, stirring occasionally, until deep golden and jammy. Tip out and set aside.

   30 g butter

3. 03

   Brown the potatoes

   Add the 1 tbsp olive oil and the remaining butter to the same pan over medium heat, arrange the potato slices in an even overlapping layer, and cook undisturbed for 5-6 minutes until golden underneath. Those crisp faces survive inside the finished frittata as golden layers. Stir them about now and you lose them.

   1 tbsp olive oil

   Do not stir the potatoesOnce they are laid out, leave them. The crisp undersides are the whole point.

4. 04

   Mix the eggs

   Whisk the eggs with the milk, 1/2 tsp Dijon, 1 tsp thyme, the salt and pepper and half the 120 g cheddar, then stir in the caramelised onion.

   8 large eggs · 2 tbsp whole milk · 120 g mature cheddar

5. 05

   Set it gently

   Pour the egg over the potatoes, scatter the remaining cheddar on top and cook over low heat for 5 minutes, until the edges are set and the centre is still liquid. Low heat matters. High heat makes the base rubbery before the middle moves.

   Pull it earlyIt keeps cooking in the hot pan. Just set is silky; firm in the pan is rubbery on the plate.

6. 06

   Finish and rest

   Transfer to a 190 C oven, or under a medium grill, for 6-8 minutes until puffed, blistered on top and only just set. The centre should still have the faintest wobble when you shake the pan. Rest for 5 minutes; residual heat finishes the eggs without turning them tight.