
Heat a dry pan to medium heat and add the chilies in a single layer. Toast 1-2 minutes per side until the skins darken slightly and become fragrant. Do not burn, as they can become bitter.
Transfer the toasted chilies to a large bowl and cover with hot water. Steep 20 minutes until softened. Reserve the soaking liquid if you wish. See Recipe Notes.
In the same pan, heat oil over medium heat. Add onion and tomatoes and cook 5 minutes until softened. Add garlic and cook another minute, stirring.
Add the onion mixture to a food processor or blender. Pull the softened chilies from the soaking liquid and add them.
Add the dried spices, apple cider vinegar, and 1 cup beef stock. Process until smooth. The sauce should be fairly thick. Strain for a smoother sauce, if desired.