Preheat the oven to 220C/Gas7; dry fry the mince until brown; add the onions and cook for a further 2-3 minutes; add the mustard, stock, herbs and season; bring to the boil and simmer for 15-20 minutes, stirring occasionally; add the sweet corn and stout and cook uncovered for a further 5 minutes
Meanwhile heat the butter and oil in a large frying pan and fry the sweet potato and turnip slices for 3-4 minutes on each side, then drain
Place 6 x 10cm metal rings on a foil-covered baking sheet and fill each ring with layers of sweet potato, turnip and mince mixture equally; alternatively layer the sweet potato, turnip and mince in a large oven proof dish
Bake for 10-12 minutes, remove and transfer rings to a serving plate; leave to rest for 2-3 minutes before removing the ring; if using an oven proof dish, allow to stand for 2-3 minutes before serving
6 servings