For the dauphinoise potatoes, preheat the oven to 200C/400F/Gas 6. Generously butter four large individual ovenproof ramekins (or one large gratin dish) and stand them on a baking tray.
Tip the potatoes into a large pan with the cream, milk, garlic, nutmeg and bay leaf and season, to taste, with salt and freshly ground black pepper. Bring to the boil, then reduce the heat and simmer for 8-10 minutes, or until just tender. Drain the potatoes through a colander placed over a bowl to collect the cream mixture.
Layer the potatoes in the ramekins and pour over the reserved cream mixture. Top with the grated Gruyère and cook in the oven for 20-30 minutes, or until bubbling and golden-brown.
Meanwhile, for the lamb stock, place all of the lamb stock ingredients into a large pan, season with salt and freshly ground black pepper and cover with 450ml/15fl oz of cold water. Bring to the boil then reduce the temperature until the stock is simmering and simmer for 10 minutes. Strain through a sieve into a clean pan.
For the sauce, boil the strained lamb stock for five minutes, or until reduced in volume by half. Stir in the port and redcurrant jelly, season with salt and freshly ground black pepper, and boil for a further 4-5minutes, or until the mixture has reduced to a glossy sauce. Stir in the arrowroot paste, as necessary, to thicken the sauce. Stir in the redcurrants just before serving.
For the lamb, season the lamb all over with salt and freshly ground black pepper. Heat the olive oil in a heavy-based ovenproof frying pan over a medium-high heat and brown the lamb all over. Transfer to the oven for 14 minutes (for medium), or until cooked to your liking. Remove from the oven and set aside to rest for five minutes.
Meanwhile, for the baby leeks and broad beans, bring a pan of water to the boil and blanch the broad beans for one minute. Drain well, then remove the membranes from the beans to reveal the bright-green inner beans.
Boil the leeks in a pan of salted water for two minutes, or until just tender. Add the broad beans for the last 30 seconds of cooking, then drain both.
To serve, carve the lamb into individual cutlets and divide among four warm serving plates. Turn out the dauphinoise potatoes from their ramekins and place next to the lamb, then arrange the leeks and broad beans alongside. Pour over the sauce and serve.