Brown roast and garlic in oil in skillet. Place in slow cooker. Add onions and peppers. Combine salsa, tomatoes and taco seasoning and pour over roast and vegetables. Cover. Cook on Low for 8 to 10 hours; cool completely.
To make the filling, shred the cold beef and heat with vegetables from slow cooker and 1/2 cup of the broth. Add chili powder, cumin and salt to taste. Simmer until all ingredients are well combined.