Slow-Cooked Beef Barbacoa

7-8 lb boneless beef round bottom roast*
slow cooking sauce:
1 1/2 onions, rough chopped
10 +/- cloves garlic
1/2 cup lime juice
1/4 cup cider vinegar
three 6 oz can tomato paste
two 16 oz jars chipotle salsa
one 7.5 oz can chipotle peppers in adobo sauce
1 Tablespoon cumin
1 Tablespoon cinnamon
1 Tablespoon oregano
1 Tablespoon smoked paprika
1 teaspoon ground cloves
1 Tablespoon salt
2 cups beef broth

In a food processor or blender, combine onions, garlic, lime juice, cider vinegar, tomato paste, salsa, chipotle peppers, spices (cumin, cinnamon, oregano, smoked paprika, cloves) and salt. Pulse to a thick, smooth consistency. Add this to a large pot with a tight fitting lid, then add the beef broth, stir to combine, and place the beef roast into the pot. Turn burner onto medium-high, until the liquid comes to a nice, quick simmer, then lower heat and cover. Keep at a low simmer for 6-7 hours, until beef can easily be shredded with a fork. Carefully remove beef from the sauce, and shred using 2 forks. Then, return shredded beef to the sauce and simmer for another 1-2 hours until very tender.