Preheat your oven to 200°C (180°C fan / Gas 6) and line a large baking tray with baking parchment.
In a large bowl, combine the sausage meat, onion, garlic, parsley, thyme, and mustard. Season generously with salt and pepper and mix with your hands until evenly combined.
Unroll the puff pastry on a lightly floured surface and cut it in half lengthways to give you two long strips.
Divide the sausage mixture in half and shape each portion into a long log down the centre of each pastry strip, leaving a 2cm border along one long edge.
Brush the exposed border with beaten egg, then fold the pastry over the filling and press firmly to seal. Crimp the edges with a fork.
Cut each long roll into 10-12 pieces, roughly 3-4cm wide, and place on the prepared baking tray with a little space between each one.
Brush the tops generously with beaten egg.
Chill the tray in the fridge for 10-15 minutes.
Bake for 20-25 minutes until the pastry is deep golden brown and the filling is cooked through. Cool on the tray for 5 minutes before serving.