In a large bowl, combine 250 g plain flour with a pinch of salt.
Add 125 g cold unsalted butter, cubed, and rub in with your fingertips until the mixture resembles coarse breadcrumbs.
Gradually add 50 ml ice-cold water, mixing just until the dough comes together.
Shape the dough into a ball, wrap it in cling film and chill in the fridge for 30 minutes.
Meanwhile, heat a frying pan over a medium heat and fry 200 g smoked streaky bacon until golden and crisp.
Drain the bacon on kitchen paper and set aside.
In the same pan, fry the chopped onion for 5–7 minutes until translucent, then remove from the heat.
Preheat the oven to 180 °C (gas mark 4).
On a lightly floured work surface, roll out the pastry to about 3–4 mm thickness and line a 24–26 cm flan tin with it.
Prick the base of the pastry all over with a fork.
(Optional) To blind bake, line the pastry with baking paper, fill with baking beans and bake for 10–15 minutes. Remove the paper and beans, then bake for a further 5 minutes.
In a large bowl, beat 3 large eggs with 250 ml double cream until well combined.
Stir in 100 g grated Emmental or Gruyère cheese, a pinch of freshly grated nutmeg, and season with salt and freshly ground black pepper to taste.
Fold in the bacon and onion.
Pour the mixture into the blind-baked pastry case and bake for 30–40 minutes, or until the filling is set and golden on top.
Allow the quiche to rest for 10–15 minutes before slicing and serving.