1. Preheat the oven to 425°F (220°C) with a rack positioned in the middle.
2. Arrange lemon slices and garlic on a rimmed baking sheet and place an oven-safe wire rack on top.
3. Remove giblets and neck from the chicken cavity. Pat the chicken dry to remove excess moisture.
4. Place chicken breast-side down on a cutting board. Using sturdy kitchen shears, cut along both sides of the backbone to remove it, keeping cuts close to the spine.
5. Make two small cuts along the neck opening of the breast bone (sternum) to aid flattening.
6. Flip the chicken breast-side up and press firmly on the breastbone to flatten the bird; a crack may be heard.
7. Season the chicken generously with kosher salt and olive oil on all sides. If using, gently separate the skin from the breast, thighs, and drumsticks, then spread about two-thirds of the flavored butter mixture under the skin and the remaining on top.
8. Place the chicken skin-side up on the wire rack above the lemon and garlic.
9. Roast in the oven for 40 to 45 minutes, or until an instant-read thermometer inserted into the thickest part of the thigh reads 165°F (74°C).
10. Remove the chicken from the oven and let it rest for 10 minutes before carving.