Preheat the oven to 350 degrees F (175 degrees C).

Using 2 fingers, loosen the skin on each chicken breast. Thinly slice compound butter and stuff under skin to cover as much of the chicken as possible.

Layer onion slices on the bottom of a glass baking dish. Pour in chicken stock. Place chicken breasts on top of the onions, skin-side up. Season with salt and pepper.

Bake in the preheated oven for 30 minutes. Start basting chicken with its juices and rotate if skin is not crisping evenly. Continue baking until chicken is no longer pink in the center and the juices run clear, about 25 more minutes, depending on the size of the chicken breasts. Baste chicken every 5 to 10 minutes. An instant-read thermometer inserted into the center should read at least 165 degrees F (74 degrees C).