Preheat the oven to 190°C, fan 170°C, gas 5. For the pastry, whiz the flour and 55g butter together in a food processor until the mixture resembles fine breadcrumbs. Add most of the yolk and water mixture, and pulse to form a soft dough – add more of the yolk and water if the mixture is too dry. Remove the dough, wrap in clingfilm and chill in the fridge for 30 minutes.
Roll out the dough and use it line a rectangular tart tin (12x35 x 2.5cm). Chill in the fridge for 20 minutes.
Remove the tart tin from the fridge and cover the pastry with baking parchment, then weigh down with baking beans or uncooked rice. Bake for 15 minutes, then remove the paper and beans or rice and bake for a further 5 minutes. Remove from the oven and reduce the temperature to 180°C, 160°C fan, gas 4.
For the filling, melt butter in a pan, add the leeks, mushrooms and chopped sage and cook for 10 minutes until soft. Fill the partly baked pastry case with the leek and mushroom mixture.
In a bowl, gently whisk together the egg, crème fraîche, milk, half the stuffing mix and half the Gruyère. Season with freshly ground black pepper and pour into the tart case.
Scatter the remaining stuffing mix and Gruyère over the top of the tart and bake for 20-25 minutes until set and golden brown. Leave to cool slightly in the tin, then remove.
Meanwhile, heat the oil in a frying pan and fry the whole sage leaves for 10 seconds, then place on a paper towel to absorb any excess oil. Arrange on top of the tart, and serve with the cranberry sauce on the side