Heat a large pot with heavy bottom on medium heat.
Dice 1 small yellow onion and peel and mince or crush 4 cloves of garlic. Add olive oil to the hot pot, and then add the onions and let saute until they are starting to turn translucent. Hold the garlic back so that it does not burn and turn bitter.
Next take the pork shoulder and cube it into 1" pieces. It is best to do this when the meat is cold. Wait to take this out of the refrigerator until just before you need it. Add the garlic and heat through, then add the cubed pork pieces to the pot. Spread the pork out over the bottom of the pan. Let the pork stand and brown a bit before stirring. It is important to sear the pork to hold in the juices.
Season the pot with salt,pepper, ground cumin, and ground coriander. Stir to let it brown on all sides. Next add the whole canned tomatillos, dice green chilies, fresh cilantro and water. The cilantro can be rinsed and just torn into small pieces.
Give everything a gentle stir. Cover over and turn to low. Let simmer for 30 minutes. After 30 minutes, take off the lid and stir/break up all of the remaining tomatillos. Then let simmer with the lid off for 30 more minutes. Pork should be tender enough to break up with a spoon and the sauce should have some liquid, but will have reduced considerably.
Serve with warm tortillas and Spanish rice.