Preheat your oven to 200°C. 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. Peel the potatoes and slice into 1cm thick rounds. Trim the root and dark green leafy part from the leek. Halve lengthways then thinly slice widthways. Peel and grate the garlic (or use a garlic press). Trim and halve the green beans. Pick the rosemary leaves from their stalks and roughly chop (discard the stalks). Once boiling, add the potato slices to the water and simmer until just tender, 8-12 mins. TIP: The potatoes are cooked when you can easily slip a knife through them.

Meanwhile, heat a drizzle of oil in a large frying pan on medium-high heat. When hot, add the leek, half the rosemary and a pinch of salt. Cook, stirring, until the leeks are soft, 4-5 mins. Add the garlic and cook for 1 min. Stir in the creme fraiche and bring to a simmer, then remove from the heat. When the potatoes are cooked, drain in a colander, then add to the leeks. Turn carefully to combine, then spoon into an appropriately sized ovenproof dish. Sprinkle the blue cheese on top of the gratin and bake in the oven until golden and bubbling, 12-15 mins.

Wipe out the leek pan and return to medium-high heat with a drizzle of oil. Season the lamb with salt and pepper then sprinkle over the remaining rosemary. Once hot, add the lamb to the pan and cook until browned all over, 2-3 mins total. Then 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. 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.

When the lamb is cooked, transfer to a plate and leave to rest a couple of mins, covered loosely with foil. While the lamb rests, pop the (now empty) frying pan on medium heat with a drizzle of oil. Once the pan is hot, add the green beans and a splash of water, then stir-fry until tender, 4-5 mins. Add the spinach and cook until wilted and piping hot, 2-3 mins. Season with salt and pepper, then use a spoon to squeeze out and discard any excess water from the spinach.

Reheat your sauce if needed. Spoon the green beans and spinach onto your plates, then thinly slice the lamb loins and plate alongside. Serve with a generous portion of gratin, spooning the red wine jus over the lamb to finish. Enjoy!