Light 1 side of grill, heating to 350° to 400° (medium-high) heat; leave other side unlit. Let slabs stand at room temperature 30 minutes. Remove plastic wrap.
Place slabs over unlit side of grill, stacking one on top of the other. Grill, covered with grill lid, 40 minutes. Rotate slabs, moving bottom slab to top; grill 40 minutes. Rotate again; grill 40 minutes.