Preheat oven to 425ºF. Place 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 chicken around & then cut along the other side of the backbone. Discard the backbone or save it & 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 on the breastbone & flatten the chicken.
Arrange lemons in a single layer on a baking sheet. Place the chicken skin side up on the baking sheet on top of lemons.
Gently use your fingers to separate the chicken meat & the chicken skin, being careful not to tear it. Slip chunks of butter under the skin.
Rub the skin with the olive oil & garlic then season with salt & pepper.
Roast the chicken until the skin is golden brown & crispy & the temperature of the thickest part of meat is 165ºF (~35-40 min).
Remove from oven & let rest on a cutting board (at least ~10 min before serving).