For the pastry, blend the flour and butter in a food processor until the mixture resembles fine breadcrumbs. Add the egg and pulse until the mixture just comes together as a dough.
Divide the pastry into eight portions and shape into balls. Flatten a pastry ball slightly and roll out on a lightly floured surface to a 3mm/½in thickness. Use the pastry to line a loose-bottomed individual tart tin. Trim any excess pastry and prick the base with a fork. Repeat with the remaining pastry balls and chill in the fridge for 30 minutes.
Preheat the oven to 200C/400F/Gas 6.
Line each tart case with greaseproof paper and baking beans and bake in the oven for 20 minutes. Remove the paper and beans and return to the oven for a further 10 minutes, or until the pastry is pale golden-brown.
Remove the tart cases reduce the oven to 170C/325F/Gas 3.
Melt the butter and oil in a large non-stick frying pan. Fry the pear slices for 1-2 minutes on each side, or until lightly browned on both sides. Remove from the pan and set aside to drain on kitchen paper.
Beat the eggs in a bowl until smooth, then stir in the crème fraîche and sage. Season with a good pinch of salt and lots of freshly ground black pepper.
Divide the Stilton amongst the tartlet cases. Add the pear slices and scatter with the walnut pieces. Slowly pour over the egg mixture, topping up each tart case when the liquid has had a chance to settle.
Bake for 25-30 minutes or until the filling is just set. Remove the tarts from the oven and set for 15 minutes before carefully removing from the tart tins.