Roast hazelnuts - Preheat oven to 180°C/350°F (160°C fan-forced). Roast the hazelnuts for 8 minutes. Cool, then roughly chop.

Blitz the mushrooms in a food processor, scraping down the sides as needed, until the mushrooms are chopped into little 3mm/0.1" pieces.

Duxelle - Melt the butter in a non-stick pan over high heat. Add eschalots, garlic and mushrooms. Cook, stirring regularly, for 5 minutes or until the liquid that comes out of the mushrooms mostly evaporates. Add spinach and cook until wilted.

Cool - Transfer the filling into a bowl. Cool for 20 minutes then stir in remaining ingredients.

Stuffing log - Pile the stuffing on a piece of cling wrap. Use your hands to roughly shape it into a 35cm / 13.5" long log. Tightly roll up with cling wrap. Place in the freezer for 3 hours (up to 3 days) until it is firm. (Note 7)

Pound - Place the lamb shoulder fat-side down. Cover with plastic wrap (I use go between) or baking paper. Then use a rolling pin or meat mallet to pound the meat (hard!) into a 40 x 22cm rectangle (15.5 x 8.5"). Aim for even thickness ~1.75cm (2/3"). Trim and patch as needed to make your rectangle - see video, I cut & rearrange 2 large flappy bits that stick out.

Roll - Sprinkle the surface of the lamb with half the salt and pepper. Unwrap the frozen stuffing log then roll it up tightly, finishing with the seam side down.

Tie - Secure with kitchen twine tied every 2cm / 0.8". (See video for tips)

Preheat oven to 220°C/425°F (200°C fan-forced).

Season - Rub the lamb roll all over with the olive oil, then sprinkle with the remaining salt and pepper.

Potatoes - Toss the potatoes with the salt, pepper and olive oil in a large roasting pan (not a baking tray). Put the lamb on top of the potato, on the diagonal if needed to fit.

Short high temp roast - Roast for 30 minutes (renders some fat and gives the colour a head start).

Slow roast - Turn the oven down to 160°C/320°F (140°C fan). Cook for 2 1/2 hours, or until the internal temperature is 98°C/208°F or the meat can be fairly easily teased apart using two fork (check in a discreet spot).

Rest - Remove lamb and potato to a tray and loosely cover with foil while you make the gravy.

Drippings - The roasting pan will have meat juices and fat. Tilt the pan so you can scoop off and discard all but 3 tablespoons of fat (the clear liquid that rises to the surface). Make sure you keep the dark brown juices in the pan. If you're short of fat, add butter.

Heat fat - Put the pan directly on the stove over medium heat. Once the fat is hot, sprinkle the flour over and stir for 1 minute.

Thicken - While stirring, slowly pour in the stock. Add soy sauce, then keep stirring as it bubbles gently until it thickens into a maple syrup consistency.

Taste and add more salt and pepper if desired (I don't need more). Pour into a jug (strain, if needed).

Cut the string off the lamb then slice into 2cm / 0.8" thick slices.

Plate on a platter, surround with the potatoes. Serve with gravy, sprinkled with extra chopped pistachios if desired! (Shown in the video with sautéed green beans).