Preheat the oven to 200°C/400°F/Gas mark 6.
Cook the sausages and onion slices in a frying pan for 6-8 minutes, until starting to brown. Sprinkle in the flour then stir in the stock. Bring to a simmer until the sauce thickens slightly.
Spoon the sausage and onion mixture into a pie dish. Cover the pie dish with the puff pastry and glaze with the beaten egg.
Bake for 20 minutes until golden. Serve immediately.
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