For the chili sauce, begin by toasting the chiles on a cast iron skillet on high, being careful not to burn them. The chiles toast quickly, as they are toasting, flip with a tongs to toast all sides.
Rehydrate the chiles in a bowl of very hot water for about 20 minutes.
On the same hot skillet, toast the onion and garlic to slightly char the outside.
Remove the chiles from the water and remove the stems.
Place the onion, garlic, and chiles into the blender with a 1/4 cup of the hot water the chiles were soaking in.
Blend until very smooth.
Pour through a strainer into a medium saucepan to remove all the chiles skins and seeds.
Add the tomato sauce, cumin, coriander, sea salt, and Mexican oregano to the saucepan.
Once the meat is ready to be shredded, turn off the crockpot, remove, cover and rest for about 10 minutes.
Using the strainer, pour 2 cups of beef broth from the crockpot into the chiles sauce. Heat on medium low for about 15-20 minutes. Save any remaining strained beef broth for the next day.
Return the shredded beef to the crockpot.
Pour the chiles sauce over the shredded beef and mix with a spatula to cover all the meat with the chiles sauce.
Place the crockpot in the refrigerator overnight to continue the next day.