Combine the brown sugar, lemon pepper, paprika and olive oil in a bowl. Use 1 tablespoon to lightly brush all over the lamb. Cover and refrigerate 1 hour if time permits. Juice 1 lemon and cut the other into wedges.
Add the lemon juice to the remaining spice mixture with the mushrooms, stir to coat. Cover and set aside 15 minutes. Meanwhile, microwave or blanch beans until bright green and tender then refresh in a bowl of iced water. Drain and pat dry with paper towel.
Preheat barbecue plate on medium-high heat. Barbecue the lamb for 4 minutes each side for medium or until cooked to your liking. Transfer to a plate, cover and allow to rest 10 minutes. Barbecue mushrooms and lemon wedges in batches for 2-3 minutes each side until tender.
Slice the lamb and arrange on plates with the mushrooms, beans, mint and macadamia nuts. Serve with barbecue lemon wedges and crusty bread.