In a 6-quart slow cooker, layer the onion, potatoes, chayote and short ribs in that order. In a blender, combine the cleaned chiles, garlic, Worcestershire, 1 teaspoon salt and 5 cups water. Blend until smooth (which will take a couple of minutes unless you have a high-speed blender). Pour through a medium-mesh strainer into the slow cooker—the liquid should nearly cover the short ribs; if not, add more water. Nestle in the epazote if you’re using it. Cover and turn on to high. Your mole de olla will be done in 6 hours, though you can hold it for longer. (My slow-cooker can be programmed to switch from high after 6 hours to a keep-warm low temperature for up to another 6 hours. Some slow cookers click to keep-warm automatically; others need to be switched manually.)
When you’re ready to serve, place the green beans in a microwaveable bowl, sprinkle with a little water, cover with plastic, poke a few holes in the top and microwave at 100% for 90 seconds, until just tender. If using the sour prickly pear fruit, cut it into small pieces: Holding them with tongs or a towel, cut the ends off the prickly pears, make a slit down the side from end to end, then peel back the thick skin, revealing the nugget of fruit. Cut it in half and, with a small spoon, scoop out and discard the reddish seedpod in the center. Cut the light-colored flesh into ¼-inch pieces.
Fish out the short ribs from the slow cooker. The meat will fall from the bones: discard the bones and tear the meat into bite-size pieces. If a lot of fat has rendered and collected on the top of the broth in the slow cooker, skim it off. Stir the meat, green beans and optional xoconostle into the soup. Taste and season with salt (usually a generous teaspoon).
Ladle into warm, deep bowls and carry to the table. Pass the lime wedges separately for everyone to add to their liking.
No Slow Cooker? In a large (6- to 8-quart; at least 12-inches in diameter) heavy pot (preferably a Dutch oven) layer the ingredients as described. Bring to a gentle simmer over medium, set the cover in place and braise in a 300-degree oven 2 ½ to 3 hours, adding water occasionally if necessary to ensure the liquid stays at about the same level. Finish with the coarsely shredded meat, green beans and optional xoconostle as described in the recipe.