Preheat the oven to 200 C / Gas 6. Line a 25 x 36cm baking tray with greaseproof paper
Roll out the puff pastry large enough to fit the baking tray, and place it on the prepared tray
Mix the onion, sausage meat, cheese, tomato puree, breadcrumbs, the beaten egg and salt and pepper to season all in a bowl with your fingers
Lay the mixture in the centre of the pastry in a long sausage shape
Dampen the edges of the pastry with the milk and seal. Turnover the pastry so that the join is underneath. Make some diagonal cuts in the top of the pastry all the way along and then brush with milk to glaze.
Bake the pie in the preheated oven for 35 to 40 minutes. Serve immediately with fresh vegetables.