Pre-heat the oven to 400ºF
Make the lemon pepper butter by combining 4 tbsp softened butter with the zest of 1 lemon, 2 cloves grated garlic, and a generous amount of fresh cracked black pepper and a pinch of salt. Mix until fully combined. Rub the lemon pepper butter all over the spatchcocked chicken and underneath the skin of the breasts.
Drizzle about 4 tbsp olive oil on a large sheet pan, then place the halved leek segments and the large lemon slices on the sheet pan. Place a wire rack over the leeks and lemons, then place the spatchcocked chicken on the rack, breast side up. Season the top of the chicken with a bit more pepper and a generous pinch of salt along with a quick drizzle of olive oil.
Roast in the pre-heated oven for 35 minutes at 400ºF, then lower to 300ºF for 40 minutes, or until a meat thermometer inserted at the thickest part reads at least 155ºF.
Meanwhile, add thinly sliced fennel and orange segments to a mixing bowl. Add 2-3 tbsp olive oil, 1 tbsp red wine vinegar, lots of black pepper, and a pinch of kosher salt. Mix until fully combined. Optionally mix in some of the fennel fronds for added color.
To serve, carve the roasted chicken and serve your desired piece (breast, thigh, leg) along with 1-2 leek segments and a few spoonfuls of the fennel orange salad. Sprinkle with flaky salt and garnish with any remaining fennel fronds.