Heat oven to 500ºF. Wrap a brick in aluminum foil and place into the oven to heat.
Place one game hen breast-side down on a cutting board. Using kitchen shears, cut from the neck to the tailbone to remove the backbone. Once you remove the backbone, you will be able to see the inside of the bird. Make a small slit in the cartilage at the base of the breastbone to reveal the keel bone. Grab the bird with both hands on the ribs and open up like a book, facing down towards the cutting board. Remove the keel bone. Cut small slits in the skin of the bird behind the legs and tuck the drumsticks into them in order to hold them in place. Season on both sides with salt and pepper. Repeat with second hen.
Fry the bacon in a 12-inch cast-iron skillet over medium heat. Once crisp, remove the bacon from the skillet and reserve. Drain all but 1 tablespoon of fat from the pan. Immediately add the two birds to the pan, skin-side down. Add the onions to the pan around the edges. Top the birds with the brick and allow to remain on the heat for 5 minutes. Place the pan into the oven and cook until the thigh meat reaches 170ºF, 10 to 15 minutes. Remove from the oven and allow the bird to rest for 5 minutes before serving with the onions and bacon.