Preheat the oven to 190ºC/375ºF/gas 5.
Lay out your shoulder of lamb. What you want to do is delicately flavour the meat, so pound up the coriander seeds with the rosemary and a pinch of salt in a pestle and mortar (or use a metal bowl and a rolling pin) until nice and fine. Take half this mixture and rub it over the inside of the lamb. Season it well with salt and pepper, then roll up the lamb and secure it with 4 or 5 pieces of string. The reason for doing this is to make the meat a consistent thickness – don't worry about doing it neatly, as long as it holds together it's fine.
Put a high-sided roasting tray on the hob and brown the lamb on all sides in a little olive oil. Remove from the heat, allow the lamb to cool a little, then add the red onions to the tray. Lift up the lamb, stir the onions around to cover them in all the flavoursome juices, then sit the lamb back on top and cook in the preheated oven for around 2 hours, adding the cranberry juice after the first half an hour and turning the heat down to 170ºC/325ºF/gas 3.
Turn the meat in its cooking juices when you can. By the end of the 2 hours you want the meat to be nice and crisp on the outside but really melt-in-your-mouth and tender. Sometimes the lamb may need a little longer, depending on the age of the animal. You'll also want enough juice left in the bottom of the tray to give everyone a nice spoonful – if it looks as if the liquid is going to cook away too quickly, add a little water and place a cartouche on top. Skim off any fat that cooks out of the meat.
While the lamb is cooking, rub the butternut squash with the rest of the spice mix and a drizzle of olive oil. Lay it in another roasting tray, season well and put it in the oven when the lamb's been cooking for just over an hour. Cook for around 45 minutes, until sweet and tender.
When the lamb's cooked, let it rest for 10 minutes, then remove the string. In a bowl mix together the coriander leaves, spring onions, lemon juice, 4 tablespoons of olive oil and seasoning. Toss together.
To serve, divide the squash between your plates, cut up the lamb into irregular sized slices, spoon over some tray juices, and sprinkle on the herb salad. A dollop of yoghurt on top is lovely.