Heat the oven to 210c. Fan 190c
Heat the olive oil over a medium high heat in a pan and brown the pancetta until crisp. Scoop from the pan and set aside. Season the chicken and add to the pan, increase the heat and brown all over.
Add the onion and leek to the pan and fry for 10 minutes until softened. Add the garlic fry for a minute more.
Add the chicken stock and spinach and season well and allow to wilt. Stir in the pesto and creme fraiche and the crispy pancetta. Mix a little of the sauce with the cornflour to form a thin paste then add this to the pan, mix well and bubble for 2-3 minutes to allow it to thicken slightly. Turn off the heat and transfer to an ovenproof dish.
Meanwhile, coarsely grate the potato and put in a bowl. Pour over boiling water then drain and tip into a tea towel. Squeeze out as much liquid as you can then tip into a bowl and toss with the melted butter and cheese and plenty of seasoning.
Spread out the rosti in a thinish layer over the top of the pie. Cook in the oven for 30-35 minutes until golden and crisp.