Preheat oven to 425ºF (220ºC). Place the chicken breast-side down on a clean workspace with the legs facing towards you.
Starting at the thigh, cut along one side of the backbone with kitchen shears. Turn the chicken around and then cut along the other side of the backbone. Discard the backbone or save it and use it for homemade stock. Flip the chicken over so the skin side is facing up. Use the palm of your hand to press down and flatten the chicken.
Arrange the lemons in a single layer on a baking sheet. Place the chicken skin side up on the baking sheet on top of the lemons.
Gently use your fingers to separate the chicken meat and the chicken skin, being careful not to tear it. Slip the chunks of butter under the skin.
Rub the skin with the olive oil and garlic then season with salt and pepper.
Roast the chicken until the skin is golden brown and crispy and the temperature of the thickest part of meat is 165ºF (74ºC), about 35 to 40 minutes.
Remove from oven and let rest for at least 10 minutes before serving.