Heat oven to 200C/180C fan/gas 6. In a shallow casserole dish, melt the butter over a medium heat on the hob. Add the onion and celery, and fry gently to soften. Add the flour and stir to coat the vegetables. Slowly add the milk, stirring really well after each addition, making sure there are no lumps of flour before you add the next splash of milk. Once all the milk has been added, bring to a simmer to thicken, then fold in the ham hock and the tarragon. Season to taste, then add about half the beaten egg.
Cut a circle of puff pastry 2cm wider than the top of the pan and pop it onto seal in the filling underneath, folding the edges to create a crust. Cut the off-cuts of pastry into rough strips and use them to decorate the top. Brush the pastry with the remaining egg and bake in the oven for
25 mins or until the pastry is golden and crisp. Serve with peas.