Preheat oven to 350 degrees. Butterfly chicken breasts so that each piece is no more than 1 1/2 inches thick. Melt butter in a large saucepan or skillet over medium-low heat, then add onions.
Once onion begins to soften and become translucent, add two or three pieces of chicken breast to pan. Cook each side for three minutes, then transfer to a foil-lined baking pan. Repeat until all chicken has been seared.
Place caramelized onions over top of chicken breasts. Place pan in the oven and bake for 17 minutes or until cooked through. Serve hot.