Preheat the grill to medium-high heat.
Add the chopped rosemary, olive oil, lemon juice, salt, and pepper to a large resealable plastic bag.
Using kitchen shears, cut down both sides of the chicken's backbone and remove the backbone.
Oil the grill pan, remove the chicken from the marinade and place the chickens skin-side down on the hot grill rack.
Place 2 foil-covered bricks on top of the chicken to weigh it down. Grill 20 minutes, then remove the bricks and flip. Continue cooking for 15 to 20 minutes longer, or until the internal temperature reaches 70°C. Let the chicken rest for 5 minutes before carving.
Serve the chicken on a bed of rocket with lemon slices.