Rinse the dried corn husks under running water. Transfer them to a bowl of hot water and weigh them down so they stay submerged. Let them soak while you prepare the sauce, meat, and masa.
Place the pork in a large pot and cover it with enough water to fully submerge the meat. Add the onion, garlic, and salt to the pot to season the pork as it cooks.
Turn the heat to high and bring the water to a boil. Once it begins to boil, skim off any foam that rises to the surface to keep the broth clear.
After skimming, reduce the heat to medium-low to let the pork simmer gently. Cover the pot partially with a lid and cook the pork for 1 hour and 40 minutes to 2 hours, or until very tender.
Check occasionally to make sure the pork remains covered with water, adding more hot water as needed.
When the pork is tender enough to shred easily with a fork, turn off the heat and let the meat cool completely in its cooking broth; this helps keep the pork juicy and flavorful. Shred the meat and strain the broth—reserve 9 cups of it for the masa.
While the meat is cooling, prepare the green sauce. In a large pot, bring water to a boil, then add the serrano and jalapeño chiles and cook until almost tender, about 10 to 15 minutes.
Add the tomatillos, onion, and 2 garlic cloves, and simmer until the tomatillos soften and change color, about ten minutes, taking care not to let the tomatillos burst.
In a blender, add half of the cooked tomatillos, all of the chiles, cooked onion, one raw garlic clove, and the two cooked garlic cloves, and blend until smooth.
Then add the remaining tomatillos and the cilantro, blending again until the salsa is smooth and thick without adding extra water.
In a large saucepan, heat the oil until shimmering, then pour in the blended salsa verde. Season with salt and simmer until thickened, about fifteen minutes.
Prepare the masa by beating the lard with a hand mixer, along with the salt and baking powder, until very light and fluffy, similar to meringue.
Add the masa harina, alternating with the hot pork broth, mixing by hand until the mixture is very smooth and resembles thick cake batter; if it is more like a paste, add additional broth and continue mixing until the masa reaches a thick cake batter-like texture.
(Test the masa by dropping a small piece into cold water—if it floats, it’s ready.) Adjust seasoning; the masa should taste slightly salty.
Pat dry a soaked corn husk. Spread about ⅓ cup of masa on the smooth side of the husk, extending the masa close to the edges. Add shredded pork and a spoonful of green sauce.
Fold both sides toward the center, then fold the bottom up to seal. Optional: tie each tamal with a strip of corn husk to keep it from opening during cooking.
Fill the bottom of a steamer (tamalera) with 1 to 2 inches of water and add two coins. (When the water is boiling, the coins will rattle; if the sound stops, the water level is low and needs more hot water.)
Line the bottom of the steamer with corn husks, then arrange the tamales upright. Continue filling the steamer until all the tamales are inside.
Cover the tamales with more corn husks and then with a clean kitchen towel. Place the lid on and cook over the stove for at least one hour, starting from the moment the water begins to boil.
After 1 hour and 15 minutes, check a tamal. A tamal is ready when it releases easily from the husk. Let the tamales rest for a few minutes. Serve warm.