Heat the oil in a large heavy-based frying pan and cook the onion for 3–4 minutes until softened.
Meanwhile, place the corned beef in a bowl, break it into chunks using a fork and mix with the mustard. Add to the pan with the onion and cook, gently stirring, for 2–3 minutes until warm, adding the cherry tomato halves for the final minute. Sprinkle over the parsley and gently toss.
Cut the pastry into 4 x 12 cm (5 inch) squares and 4 x 10 cm (4 inch) squares (rolling the pastry a little wider first if necessary. Use the larger pieces to line a 4-hole Yorkshire pudding tin with the corners overlapping the hole edges.
Divide the corned beef and tomato mixture between the pastry cases. Lightly brush the edges of each pie with water and place the smaller squares on top as lids. Press down well to adhere, and make a hole in the centre of each. Brush with beaten egg and bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 20 minutes until golden and puffed. Serve.