In a small food processor combine oil, onion, garlic, allspice, cumin, chilli flakes, juice and zest of 1 lime, sugar and thyme. Season and process until smooth.
Place lamb into a large snap lock bag or glass dish, pour in marinade and rub to coat. Refrigerate for 1 hour or overnight, if time permits.
Heat a large lightly oiled char-grill pan or barbecue over medium-high heat. Drain lamb from marinade and cook lamb for 10 to 12 minutes on each side, turning regularly to prevent burning, or until cooked to your liking. Set aside to rest for 10 minutes before slicing.
Meanwhile, in a medium bowl place pineapple, remaining lime juice and zest, chilli and chopped mint. Season and stir to combine.
Serve sliced lamb with pineapple salsa, sweet potato fries, greens and mint sprigs.
Omit chilli from recipe if making for children or people who do not like spice.