Preheat oven to 400 degrees. Double wrap brick in aluminum foil and place in the oven.

Butterfly the chicken placing the chicken on a cutting board breast side down. Use kitchen shears or a sharp knife to cut out the backbone of the chicken. Place your hands on top of each breast and press down hard to make it as flat as possible. In a bowl mix together the rosemary, garlic, butter, cayenne pepper, dried onion, and lemon juice. Place in the microwave for 15-20 seconds. If the butter is not completely melted use a whisk to melt it down completely and to blend all the ingredients. Generously season chicken with salt and pepper then brush half the mixture on the back of the chicken. Flip the chicken and repeat, season, and brush on the mixture.

Heat oven-safe skillet (cast iron or grill pan) medium-high heat. Add in vegetable oil and once you see light white smoke rise from the pan it is ready. Place the chicken in the skillet, skin side down, along with any remaining pieces of rosemary; weight it with 1-2 aluminum wrapped bricks or another heavy cast-iron skillet. The idea is to flatten the chicken by applying weight evenly over its surface.

Cook over medium-high to high heat for 8-9 minutes and check to make sure chicken is a mahogany brown. Flip the chicken over (it will now be skin side up) and roast in the oven for 25 minutes more, or until done (large chickens may take an additional 5 minutes or so).