Soak two handfuls of clean hay in cold water for at least an hour. Squeeze it dry. Put half in a roasting tray and set the rest aside. Preheat the oven to 200C/180C Fan/Gas 6.
For the lamb, mix together the butter, tarragon, mint, parsley, garlic and salt and pepper to taste. Spread it all over the lamb, season well, then lay the lamb on the hay. Add the carrot, onion, fennel and rosemary. Cover with the remaining hay and a layer of aluminium foil and roast for 1 hour.
For the bubble and squeak, rub the potatoes with the oil and put them on a baking tray. Bake for 1 hour, or until the cooked.
Reduce the oven to 160C/140C Fan/Gas 3, remove the foil and some of the hay and roast the lamb for a further hour; it should almost fall off the bone. Transfer it to a platter to rest, then remove the remaining hay from the tray.
Pour the stock into the roasting tray and bring to boil on the hob. Strain through a fine sieve into a saucepan, then skim off any fat using a spoon and keep warm.
For the mint jelly, put the gelatine in a shallow bowl and just cover with cold water. Leave for a few minutes to soften. Put the sugar, apple juice and vinegar in a saucepan over a low heat until the sugar has dissolved. Turn up the heat and bring nearly to the boil. Remove the gelatine from the water, squeeze out any excess, add to the juice and stir until completely dissolved. Set aside to cool.
Add the shallot, mint and plenty of salt and pepper to the apple juice mixture, cover and refrigerate until set.
For the bubble and squeak, boil the swede in salted water until tender, then drain well. Blanch the cabbage in boiling salted water, refresh under cold water, then drain well. Melt the butter in a large frying pan over a medium heat, and sauté the shallots, garlic and thyme until lightly coloured. Add the cabbage and cook for 2 more minutes.
Scoop the potato flesh out of the skins into a bowl. Add the swede, then mash together, retaining some texture. Add this and the parsley to the cabbage mixture, season and mix well. Cook for 1-2 minutes, stirring from time to time.
Serve the lamb with the bubble and squeak, gravy and mint jelly.