In a large 9×13 baking dish, mix the lemon zest, rosemary, garlic, olive oil, salt and pepper. Stir to combine. Add in the flattened chicken, and rub it with the olive oil mixture on both sides. Allow to marinate for at least 30 minutes, or up to 24 hours.
Heat your grill over medium high heat, and oil the grill grates. Cook the chicken skin side down, and weight the chicken down with aluminum foil bricks, or the same baking dish you marinated the chicken in. Cook for 20 minutes, then flip the chicken so it is skin side up. Put the brick or baking dish back on the chicken. Continue cooking until the chicken has an internal temperature of 165 degrees, about 20-40 minutes more, depending on the size of your chicken.
In the last 10 minutes of cooking the chicken, grill the lemons flash side down.
Remove the chicken from the heat, and allow to rest for 10 minutes. Carve, and serve with the grilled lemons. Squeeze the lemons on the chicken. Leftovers will keep refrigerated for 5 days.