Place the lamb steaks in a dish with the lemon rind, oil and 1–2 teaspoons harissa and rub to coat. Arrange the coated steaks on the rack of a foil-lined grill tray and cook under a grill preheated to a medium-high setting for 5–7 minutes, turning once, until cooked but still slightly pink in the middle. Remove and set aside to rest for 1–2 minutes.
Meanwhile, stir 1–2 teaspoons harissa into the hummus. Lightly toast the pittas in a toaster, then arrange on warmed plates and top with a dollop of hummus. Arrange the lamb steaks on top of the hummus, drizzled with any juices. Serve with lemon wedges and rocket leaves, if desired.