Well season the chicken with salt and pepper. Pop a splash of oil into a large frying pan and cook the chorizo until crispy. Put the chorizo to one side, leaving the oil in the pan. Add the chicken chunks to the pan and brown all over. Set to one side.
In a separate pan, add the wine and simmer until it has reduce by about 75%, then add the chicken stock and slowly heat. To the finely grated cheese add the cornflour and make sure all the strands are covered. Add the cheese to the stock and stir occasionally until the cheese has melted - don't let this boil! Keep warm.
In a large saucepan, heat a splash of oil and cook the garlic, pepper and onions until soft. Stir in the mushrooms, parsley, tarragon, lemon juice and season really well with salt and pepper. Cook for 5-10 mins.
Add the chicken, chorizo and cheese sauce and stir well so that the sauce coats all of the ingredients. Cook for another 10mins, stirring occasionally then take off the heat.
Butter a large pie dish and roll out the pastry on a lightly floured surface so it is 1/8 inch thick. Cut the pastry into 1cm strips.
Pop the pie filling into the dish (pile it high!) and brush around the edge of the dish with the beaten egg. Place a strip of pastry vertically down the middle of the pie, then line up other strips going towards the edges, leaving a 1cm gap between the them. Then, slowly and carefully (put the wine down at this point) weave the other strips horizontally to create the lattice. Finish the pie with strips of pastry around the outside to form a frame. Once all in place brush with a coating of the beaten egg and pop in the fridge for at least 30 mins.
When ready to cook, pre-heat the oven to 320ºf / 160ºc. Take the pie out of the fridge and pop another layer of the beaten egg on top. Bake for 40-45mins until golden brown.