Unroll the pastry and cut into 4 equal rectangles. Place the sausagemeat in a bowl with the mustard and parsley and mix well. Using lightly floured hands, divide the sausagemeat into 4 pieces, then shape each into a rough square.
Place 1 square at an angle in the centre of 1 pastry rectangle, then place a tomato slice on top. Fold the corners of the pastry up and over the sausagemeat to form an envelope shape. Place on a baking sheet and brush with beaten egg. Repeat with the remaining ingredients to make 4 turnovers.
Bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 15–20 minutes until well risen and golden. Serve warm with hot baked beans, if liked.