To spatchcock the chicken: Put the chicken breast-side down on a work surface and use poultry shears to cut along both sides of the backbone. Open up the chicken and use the tip of a knife to score alongside the keel bone (the dark oblong bone in the middle of the breast); this makes it easier to flatten. Turn the chicken breast-side up and press down strongly with the heel of your hand on the center of the breast. You should hear a crack and feel the bones give way.
Refrigerate the chicken, skin-side up and uncovered, for 1 1/2 hours to air-dry. Remove and allow to air-dry at room temperature for an additional 30 minutes before cooking.
Preheat the oven to 450 degrees F.
Halve the lemons, then slice one of the lemon halves into rounds. Using your index finger, separate the skin of the chicken from the meat and slide the lemon slices underneath the skin. Season the chicken with salt and lots of freshly ground pepper.
In a large cast-iron pan, heat the oil over medium-high heat. Add the chicken skin-side down and cook until the skin is well browned, 8 to 10 minutes. Scatter the thyme and rosemary on top (leave the chicken skin-side down) and place the 3 remaining lemon halves in the pan cut-side down. Transfer to the oven and roast until the juices run clear and an instant-read thermometer inserted in a thigh registers 165 degrees F, about 40 minutes. Rest the chicken on a plate or baking sheet skin-side up for 20 minutes before carving. Squeeze the roasted lemons over as desired.