Cut the onion in half, and roughly slice each half, and place the slices at the bottom of the slow cooker in a single layer. Add water to the slow cooker. Place them on top of the onion. Cook on high.
Combine all the sauce ingredients in a small bowl. Pour the sauce to a large pan, and bring to a boil over medium heat. Using a pair of kitchen tongs, add the meat and sear. Do not burn the sauce.
If doubling the sauce, use the same pan, after removing the meat, to boil the sauce over medium heat until slightly thickened. Adjust the salt.
Transfer the meat to a cutting board. Thinly slice crosswise.