Heat the oil in a small frying pan, add the onion and fry gently for about 5 minutes until softened and translucent.
Mix the minced beef, sausagemeat, herbs and seasoning in a large bowl and stir well. Mix in the fried onion. Beat the egg with the soy sauce and add to the mixture to bind.
On a floured surface, roll out the puff pastry to a rectangle, 25 x 40 cm (10 x 16 inches). Place on a baking sheet. Mark into 3 sections lengthways and spoon the meat mixture down the centre, to within 2.5 cm (1 inch) of the short edges. Cut the pastry down each side of the meat into diagonal strips, 1 cm (1/2 inch) apart, and brush lightly with beaten egg.
Take one strip from each side of the pastry and then cross them alternately over the meat. Continue folding over the pastry strips to give a plaited effect. Dampen the top and bottom ends of the plait with a little water and fold over to seal.
Brush all over with beaten egg and then bake in a preheated oven, 200°C (400°F), Gas Mark 6, for about 40-45 minutes until the pastry is crisp and golden. Serve hot or cold garnished with watercress sprigs and tomato quarters.