Heat the oil in a medium-sized saucepan over a medium heat, then add the onion and mushrooms and cook and stir for a few minutes until soft. Add the lamb and cook and stir to brown, breaking the meat apart with a wooden spoon; pour off any excess fat. Add the red wine, garlic, tomato purée, Worcestershire sauce, honey, then Dijon and fry for a few minutes. Pour in the stock, bring to a simmer, then cover and cook for 40 minutes, uncovering halfway and adding grated carrots.
Preheat the oven to 220 C / Gas 7.
Meanwhile, to make the mash, bring a saucepan of lightly salted water to the boil and add the potatoes. Cook for 10 to 15 minutes until tender. Drain, then mash with the butter and milk, and add half of the cheese.
Put the mince into an ovenproof dish, top with the mash and ruffle with a fork.
Bake for 20 to 25 minutes until the top is starting to colour and the mince is bubbling through at the edges, then sprinkle the rest of cheese on top. Flash under the grill to brown, if you like. Leave to stand for 5 minutes before serving with mint jelly.