Preheat the oven to 190 degrees.
Boil the potatoes and cook until soft, approx 20 minutes.
Meanwhile in a saucepan over medium-high heat cook the onions in half of the butter for approx 5 minutes, until soft. If the carrots are fresh cook them with the onions, if frozen add after the beef mince is browned.
Add the beef mince, once browned add the vegetables and BBQ Master Rub. Pour in the stock, season with Wild Porcini Salt and simmer for a further 5 minutes, with the lid off.
When the potatoes are soft, remove from pot, add remaining butter and mash until soft.
In an ovenproof casserole dish, layer the beef mixture first, then
top with the potato mash in an even layer, using a fork to create peaks.
Cook in oven until brown and bubbling, approx 30 minutes.