Pulse 175g of all-purpose flour, 100g of cold butter, 1 egg yolk, and 4 teaspoons of cold water in a food processor until the dough binds.
Roll it out thinly and line a 9in / 23cm loose-bottomed fluted tart pan, 1in / 2.5cm deep. Trim and chill for 10 minutes. Preheat the oven to 400°F / 200°C.
Line the pastry with foil and pie weights and bake for 15 minutes. Remove the foil and weights and bake for 4-5 minutes more, until pale golden. Reduce the oven to 375°F / 190°C.
Fry 200g of smoked bacon lardons until lightly colored but not crisp; drain. Dice three-quarters of 50g of Gruyère and grate the rest. Scatter the diced cheese, then the lardons, over the pastry.
Beat 200ml of crème fraîche to loosen it, then slowly beat in 200ml of heavy cream, 3 beaten eggs, and 1 pinch of ground nutmeg.
Pour most of the filling into the case. Set the pan on the oven rack, pulled halfway out, and pour in the rest. Top with the grated cheese and bake for about 25 minutes, until golden and softly set.
