Layer the chopped onion into the bottom of a slow cooker. Season the pork shoulder with salt and pepper; place atop the chopped onion. Pour the green salsa over the pork. Sprinkle the the cilantro over the salsa and pork. Drop the serrano chile peppers into the slow cooker.
Cook on Low until the meat falls apart easily, about 8 hours. Gently remove the pork to a cutting board. Strain and discard about half the remaining liquid from the slow cooker, reserving the rest. Discard the onions, peppers, and cilantro if desired.
Shred the pork shoulder with a pair of forks. Mix the pork with the reserved liquid from the slow cooker to serve.
