Preheat oven to 200 c. With a sharp knife, make incisions in the lamb and push in salt of garlic. Season lamb with salt and pepper, then place in a roasting tin with sprigs of rosemary. Cook in the oven for 40 minutes.
Meanwhile, cut the squash in half, discard the seeds, then place in a roasting pan, cut side down, with enough water to come half way up sides of pan. Simmer for 15-20 minutes until squash is tender. Remove from the pan and scoop out the centre into a colander. Drain.
To make the dressings, remove core and seeds from the pepper and put the flesh into a blender or food processor with the vinegar and oil, work until smooth. Season with salt and pepper and set aside.
Remove lamb from oven stand for 5 minutes. Divide squash between 6 plates. Slice lamb and arrange on the plates. Spoon over dressing and garnish with a few peas and sprigs of rosemary.