Heat the butter in a large heavy-based casserole or saucepan. Add the gammon and turkey, cooking over a medium heat for 5-6 minutes, until evenly browned. Add the leeks, cover and cook for 3 minutes. Uncover, stir in the flour and cook for about 1 minute. Pour in the stock and bring to a brisk simmer, stirring and scraping the base of the pan, for about 4-6 minutes, until the mixture thickens.
Set aside to cool for 30 minutes before stirring in the mustard and crème fraîche. Mix in the tarragon and sprouts and transfer to a large pie dish (at least 1.5 litres in size). Leave to cool.
Preheat the oven to 220˚C/Fan 200˚C/Gas 7. Roll the pastry on a floured work surface so that it’s large enough to cover the pie dish with plenty overhanging. Cut 1.5cm strips from the outside of the pastry. Brush the rim of your dish with egg, place the strips on top and brush these with more egg. Cut a couple of slits in the pastry sheet to let steam escape (or for a pie funnel) and lay on top of the pie. Crimp the edges, brush with more egg and decorate if you wish using the pastry trimmings. Bake for 5 minutes and then reduce the oven temperature to 200˚C/Fan 180˚C/Gas 6. Cook for a further 30-35 minutes, until golden and piping hot.