Season the beef shoulder all over with salt and pepper then brown in a large heavy pot over medium heat. Once browned on all sides, add enough water to cover the roast along with the 1 sliced onion and about 6 cloves of garlic. Cook until the meat is fork tender and comes apart with no resistance, about 2 hours. When done, remove the roast to a platter to cool, reserve the beef broth. Hand shred the meat and set aside.
To prepare the sauce, remove the tops of the dried chilies and shake out most of the seeds. Place the chilies in a large stockpot and cover them with water. Add the cumin, remaining sliced onion and garlic. Boil for 20 minutes until the chiles are very soft. Transfer the chiles to a blender using tongs and add a ladle full of the chile water (it is best to do this in batches.) Puree the chiles until smooth. Pass the pureed chiles through a strainer to remove the remaining seeds and skins. Pour the chili sauce into a large bowl and add salt, stir to incorporate. Taste to check seasonings, add more if necessary. Add the shredded beef to the bowl of chili sauce, and mix thoroughly. Refrigerate until ready to use.
Go through the dried cornhusks, separate them and discard the silk, be careful since the husks are fragile when dry. Soak them in a sink filled with warm water for 30 minutes to soften. In a deep bowl, combine the masa, baking powder, and salt. Pour the broth into the masa a little at a time, working it in with your fingers. In a small bowl, beat the vegetable shortening until fluffy. Add it to the masa and beat until the dough has a spongy texture.
Rinse, drain, and dry the corn husks. Set them out on a sheet pan covered by a damp towel along with the bowls of masa dough and beef in chili sauce. Start with the largest husks because they are easier to roll. Lay the husk flat on a plate or in your hand with the smooth side up and the narrow end facing you. Spread a thin, even layer of masa over the surface of the husk with a tablespoon dipped in water. Do not use too much! Add about a tablespoon of the meat filling in the center of the masa. Fold the narrow end up to the center then fold both sides together to enclose the filling. The sticky masa will form a seal. Pinch the wide top closed.
Stand the tamales up in a large steamer or colander with the pinched end up. Load the steamer into a large pot filled with 2-inches of water. The water should not touch the tamales. Lay a damp cloth over the tamales and cover with lid. Keep the water at a low boil, checking periodically to make sure the water doesn't boil away. Steam the tamales for 2 hours.
The tamales are done when the inside pulls away from the husk. The tamale should be soft, firm and not mushy. To serve, unfold the husk and spoon about a tablespoon of remaining beef filling on top.