Place the roast in the slow cooker and add the tomatoes, onion, garlic, salt and bay leaves, then pour water over the roast. Place the lid on and set on high for 6 hours or low for 8.

After the meat has been cooking for 4 hours (if cooking on high) to 6 hours (if cooking on low), make the sauce.

Using gloves and scissors, cut the chiles open and remove the stem and seeds. Place them in a bowl and cover with boiling hot water. Let them soak for 20 minutes. You may need to place a small plate or bowl on them to weigh them down and keep them under the water.

Place the spices in a pan over medium high heat and toast for 1 to 2 minutes, or until you can begin to smell the spices. Stir often. Pour the spices into a blender.

Place the garlic in the same pan and brown each side, then pour into the blender.  

Spoon out 2 cups of the beef broth from the crockpot, and the tomatoes, and add to the blender. Drain the chilis and add them to the blender as well, then blend until smooth.

Add the vinegar, and a teaspoon of salt, and blend for 10 seconds more.

Pour the chile sauce mixture through a fine mesh strainer and press with a spoon until strained.

When the meat is tender, remove from the broth and shred. Discard garlic head from crockpot. 

Add the sauce to the slow cooker and stir into the broth. Add the meat back into the slow cooker and continue cooking for another hour.