Layer sliced onions on the bottom of the slow cooker.
Cut the roast in half, or however will make it fit best in your cooker, and place on top of the onion.
In a mixing bowl, combine remaining ingredients, then pour over the roast. If it looks like there's not enough liquid, add up to a cup of broth, stock, or your favorite beer. We want the meat to be almost covered in liquid.
Cover and cook on low until meat is cooked through and tender, about 8 to 10 hours. About 30 minutes before serving, shred the meat using two forks. This can sometimes be done in the slow cooker itself, but feel free to transfer to a cutting board if it's easier.
If you want the liquid to thicken up, cook it these last 30 minutes with the lid askew to let the liquid evaporate.
Serve on hamburger buns, with a side of coleslaw, or on tacos.