Peel the potatoes and cut into 3cm chunks. Put the potato in a medium saucepan of cold salted water and bring to the boil. Reduce the heat to medium and cook for 10-12 mins until tender. Drain, return to the pan and cover to keep warm.
Step 2 — Prep ingredients
Meanwhile, finely chop the onion and carrot. Put the cornflour, half the tomato paste*, 2 tsp white wine vinegar, 2 tsp dijon mustard and 125ml (½ cup) water in a jug and stir until combined.
Step 3 — Start filling
Heat 2 tsp olive oil in a large deep frypan over medium heat. Cook the onion and carrot, stirring occasionally, for 6-8 mins until softened.
Step 4 — Cook lamb mince
Increase the heat to high, add the lamb mince and cook, breaking up the lumps with a spoon, for 4 mins or until browned. Stir in half the umami seasoning* and cook, stirring, for 1 min.
Step 5 — Simmer filling
Preheat the grill to high. Stir the tomato and cornflour mixture and peas into the lamb mixture. Bring to the boil and cook, stirring, for 1 min. Reduce the heat to medium-high and cook, stirring occasionally, for 5 mins or until the sauce is thickened. Taste, then season with salt and pepper.
Step 6 — Grill pie and serve
Add 2 tbs milk and 20g butter to the potato and mash until smooth. Taste, then season with salt and pepper. Transfer the lamb mixture to a 1L (4 cup) ovenproof dish, then spoon over the mash and rough up the surface with a fork. Coarsely chop 20g butter and scatter over the pie. Grill for 5-10 mins until golden. Serve the lamb pie at the table.