Beef; 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.
Sauce; 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.
Husks; go through the husks and sort by size and remove any silks or debris. Soak the corn husks in your sink filled with warm water until soft, about 30 minutes. (You can also soak husks in large bowls of water if you are not doing a lot of tamales).
Masa; 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.
Drain Husks; 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.
Spread Masa; 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. The easiest method of spreading masa is with a masa spreader (see side panel for how this is used) Do not use too much, keep the masa thin.
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.
Fold; the sides of the corn husk to center over the masa so that they overlap to make a long package. Fold the empty part of the husk under so that it rest against the side of the tamale with a seam.
Steam; Place the tamales in a steamer and cook tamales for 2 hours. Check every 20 minutes. Do not let the water boil away. The tamale is cooked when it separates easily from the corn husk.
Serve; Unfold the husk and spoon about a tablespoon of remaing beef filling on top