Make the mashed potatoes.
Preheat an oven to 200°C (conventional) or 180°C (fan-forced).
Heat the oil in a large saucepan over a medium-high heat. Add the onion and cook it by continuously stirring for 5 minutes or until it becomes soft. Add the carrot and celery, and cook for a further 5 minutes.
Add the lamb mince to the saucepan and cook it over a low heat for 5 minutes (or until the mince changes colour). Use a spatula to break up any clumps.
Add the flour and stir the mixture for 2 minutes (or until the flour becomes well-combined). Add the tomato paste, stock cubes, and water. Cook the mixture for 30 minutes, and thicken the sauce as necessary by reducing it.
Once you’ve finished cooking the mixture, remove it from the heat, stir in the AdVital Powder and peas until they’re well-combined, and then season to taste.
Place the lamb mixture into a 2L casserole dish and spread the mashed potatoes evenly over the top. Bake it for 25 minutes (or until mash becomes golden). If you’re using cheese, sprinkle it over the mashed potato during the last 5 minutes of baking. Serve immediately.