I worked at McDonald's many years ago. This is the way that they made the onions for the burgers. I did this to go with Recipe #106882. I'm guessing at the yield but you can get enough for 4-5 burgers depending on how much you like funky onions.
Put onion and water in a small microwave proof bowl. Heat for 30 seconds on medium. Let sit 15 minutes or until the water has been absorbed. If the onions still seem wet you can press out the extra water with the back of a spoon. Spread onion on top half of your hamburger bun.