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