Preheat your oven to 220°C/200°C fan/gas mark 7.
Remove the lamb from the fridge to allow it to come up to room temperature.
Pop a saucepan of water with 1/2 tsp salt on to boil for the potatoes. Peel the potatoes and slice into 1cm thick rounds.
Once boiling, add the potato slices to the water and simmer until just tender, 8-12 mins.
Meanwhile, trim the green beans. Peel and grate the garlic (or use a garlic press).

Once the potatoes are cooked, reserve some of the potato water (see ingredients for amount) in a measuring jug, then drain in a colander.
Stir the chicken stock paste, creme fraiche and half the garlic into the potato water to make your creamy sauce. Season with salt and pepper.
Arrange the potato slices in an ovenproof dish and pour over the creamy sauce. Sprinkle with the cheese and bake on the top shelf of your oven until golden and bubbly, 15-20 mins.

Pop a medium frying pan on medium-high heat with a drizzle of oil. Season the lamb with salt and pepper then sprinkle over the dried rosemary.
Once hot, add the lamb to the pan and cook until browned, 2-3 mins total.
Transfer to a baking tray, fat-side up. Roast on the middle shelf for 7-9 mins. TIP: This will result in medium-rare lamb. Cook for 2-3 mins longer if you want it more well done.
When cooked, transfer to a plate and leave to rest a couple of minutes, covered loosely with foil. IMPORTANT: Wash your hands and equipment after handling raw meat. The lamb is cooked when browned on the outside.

While everything is in the oven, clean out the potato pan.
Pour in the water for the sauce (see ingredients for amount). Bring to the boil, then stir in the red wine jus paste.
Reduce the heat slightly, then bubble away until the sauce has reduced by half, about 5-6 mins.
Once glossy and thickened, remove from the heat. Season with plenty of pepper.

Meanwhile, heat a drizzle of oil in the now empty frying pan on medium-high heat.
Once the pan is hot, add the green beans and stir-fry until starting to char, 2-3 mins.
Stir in the remaining garlic, turn the heat down to medium and cook for 1 min.
Add a splash of water and immediately cover with a lid or some foil. Cook until the beans are tender, 4-5 mins. Remove the pan from the heat.

When you're ready to serve, reheat the red wine jus if needed.
Thinly slice the lamb loins and transfer to your serving plates. Serve with the garlicky green beans and a generous portion of gratin alongside.
Spoon the red wine jus over the lamb to finish. Enjoy!