Preheat the oven broiler. Set an oven rack about 7 to 8 inches from the heat source.

Spread onion slices in a single layer on the bottom of a baking dish. Season chicken with salt. Place chicken halves skin-side down over onion rings.

Cook chicken in the preheated oven, turning every 7 minutes, until caramelized, no longer pink at the bone, and the juices run clear, about 30 minutes. An instant-read thermometer inserted into the thickest part of the thigh, near the bone, should read 165 degrees F (74 degrees C).