Spatchcock chicken. Preheat the oven to 450 degrees F, ensuring a rack is positioned in the center of the oven. Pull the spatchcocked chicken from the refrigerator to give it a chance to warm to room temperature for more even roasting – the surface of the chicken should be pretty dry to the touch, if not, pat it down with some paper towel. Gather & prep ingredients.
Add all listed lemon herb butter ingredients to a medium bowl. Season with S&P as desired. Mix to combine well. Set aside for immediate use, or store in an airtight container in the refrigerator for up to 1 week.
Use your hands to spread the prepared lemon herb butter between the chicken skin & meat. Use the same technique to season the chicken legs by working your hand down the side of the chicken into the thigh and leg area. If you have any leftover lemon herb butter, rub it across the surface of the spatchcock chicken. Set aside.
Place the baby potatoes, sliced onion, & quartered lemon in an oven-safe skillet or braiser. Lightly drizzle with olive oil, then season liberally with S&P as desired. Toss to coat. Place the buttered spatchcock chicken over the potatoes (skin side facing up). Lightly drizzle the surface of the spatchcock chicken with olive oil, then season liberally with S&P as desired.
Transfer the skillet or braiser to the oven, immediately reducing the oven temperature to 425 degrees F. Roast for 1 hour - 1 hour 10 minutes, rotating the chicken every 20 minutes to promote even roasting & browning. Pull the chicken from the oven once an instant-read thermometer inserted in the thickest part of the breast & thigh register an internal temperature of 160 degrees F.
Remove from the oven & carefully tent with foil. Set aside to rest at room temperature for 10-15 minutes before carving. Carefully transfer the chicken to a cutting board to carve as desired. Discard the spent roasted lemons, using tongs to carefully squeeze any remaining juice over the roasted potatoes & onions. Serve the lemon roasted spatchcock chicken with the potatoes, onions, & any lemon herb chicken drippings in the skillet.