To make the compound butter: In a small bowl blend the butter, salt, pepper, garlic powder, onion powder, and chopped herbs. Set aside.
Preheat oven to 425℉.
Place the chicken breast side down on a sheet pan or plastic cutting board. Spatchcock the chicken by removing the backbone with sharp kitchen shears. Start at the tail end of the bird and cut up one side of the backbone all the way through the neck, repeat on the other side of the bird to fully remove the backbone. Open the bird and place the bird breast side up in front of you. Place your hands over the breast bone and firmly press down with even pressure to crack the breast bone and help the chicken lay flat.
Separate the skin from the breast by opening the skin at the bottom of the breast and using the back of a teaspoon, wiggle the spoon up between the skin and the breast meat, all the way to the top, without ripping or tearing the skin.
Place 1 tablespoon of compound butter in the pocket of skin of each breast and smooth to coat the breast meat with the butter. Pat the chicken skin dry, and cover with the rest of the compound butter.
In a medium bowl, toss the potatoes, garlic heads, lemon halves, and onion sliced with the oil, salt and pepper. Place on the baking sheet around the chicken.
Bake for 45-60 minutes, or until a thermometer placed in the thickest part of the thigh registers 165℉. Let rest for 15 minutes before carving. Squeeze out roasted garlic and toss with potatoes. Serve with lemon halves.
Refrigerating: Store leftover chicken and potatoes in an airtight container in the fridge for 4-5 days.Freezing: Cool completely and place leftovers in a freezer safe, airtight container. Store in the freezer for up to 3 months.Reheating: Reheat gently in the microwave. Chicken, especially white meat, can dry out quickly when reheated.Repurposing leftovers: Use the leftovers for pita pockets, chicken salad, wraps, tacos, soup and more. The backbone and other bones can be used to make a homemade chicken stock.