Add potatoes to a pan and cover with cold water. Put on to boil, and gently boil till cooked.
Add oil to a pan over medium heat. Add onion and cook gently for 5 minutes.
Crumble lamb mince into the onions and cook till browned, draining off any excess fat.
Crumble in stock cube and add tomato puree till mixed all in.
Boil kettle for gravy. Add gravy granules to the required amount of water and stir till smooth. Pour gravy over mince.
Add the frozen peas and carrots to the mix and simmer for 2 to 3 minutes. Then pour into the dish you are going to use and let it sit for about 30 minutes till slightly cool.
Drain and mash the potatoes in your preferred way (I always mash the potatoes to get the lumps out first before adding the butter then milk, as I find I get a smoother mash).
Add mash to the corners and edges of the dish first before filling in the middle.
Bake in a preheated 200 C / Gas 6 oven for 1 hour, or till top is golden brown.
Topping
I normally rough the potato with a fork and sprinkle grated cheese on top for a cheesy crust.