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. 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 until crisp, 10-12 minutes. Remove the bacon to a paper towel lined plate and set aside. Drain all but 1 tablespoon of fat from the pan and immediately add the two birds, skin-side down. Scatter the onions around the edge of the pan, then top the birds with the brick, allowing it to remain on the heat for 5 minutes. Place the pan into the oven (with the brick still on top) and cook until the thigh meat reaches 170ºF, 15-20 minutes. Remove from the oven and allow the bird to rest for 5 minutes before removing the brick. Serve the hens on a platter with the onions and bacon. Any juices left in the pan can be poured on top.