How to Make It
Set a chicken on a work surface breast side up. Using kitchen shears, cut between the breast halves to split them. Remove the breast bone with your hands, then flatten the chicken; transfer to a large rimmed baking sheet. Repeat with the remaining chicken.
In a mortar, using a pestle, crush the peeled garlic cloves with a generous pinch of salt until a paste forms. Scrape the paste into a small bowl and stir in the oil, rosemary and the lemon zest and juice. Spread the mixture all over the chickens and season with salt and pepper. Let stand for 1 hour.
Light a grill and set it up for indirect cooking. If using a charcoal grill, rake the coals to one side. Oil the grate. Grill the chickens skin side up and the halved garlic heads and lemon quarters cut side down uncovered over indirect heat until the chickens are browned on the bottom and the garlic and lemons are lightly charred, 20 to 25 minutes. Flip the chickens and place the garlic halves and lemon quarters in the cavity. Continue to grill uncovered over indirect heat until the skin is well browned and an instant-read thermometer inserted in the thickest part of each chicken registers 160°, about 20 minutes. Transfer the chickens, garlic halves and lemon quarters to a carving board and let rest for 15 minutes. Carve the chickens and serve with the grilled lemon and garlic.