First, cut the dried chilies and remove the stems and seeds. Then, bring a pot of water to a boil and add in the onion, dried chilies and garlic. Boil for about 15 minutes to allow the chilies to rehydrate and to soften the onions and garlic.
Now, drain the water from the boiled ingredients. Add to a blender with the tomato sauce, adobo sauce and peppers. Blend until smooth.
Pour the contents into the crockpot. Then, add in brown sugar, oregano and salt. There are no measurements for the salt because that will depend on the size of the chuck roast. For this recipe I used about 2.5-3 pounds.
Cook the chuck roast on high for 6 hours or low for 8 hours. The chuck roast will be super tender when it's finish and not need to worry about it drying out. If you need to add more time no need to worry.
Allow to cool a bit before making the tacos. Before making the tacos be sure to do your final taste for salt after shredding the beef.