To prepare BBQ chicken:
Place chicken breasts in a slow cooker. In a medium mixing bowl, combine BBQ sauce, Worcestershire sauce and liquid smoke; pour over chicken. Cook the chicken on HIGH for 4 hours or LOW for 8 hours.
When the chicken is done, pull chicken out of the slow cooker and shred with two forks. Place the chicken back in the slow cooker to absorb more of the barbecue sauce while preparing the rest of the dish.
To prepare sauce:
In a medium saucepan heat oil and saute onion and garlic clove until tender. Dissolve bouillon in hot water then add to saucepan. Stir in tomato sauce, barbecue sauce, chili powder, sugar, salt and cumin.
In a cup, combine 1/2 cup cold water and cornstarch, stir well. Gradually, stir cornstarch mixture into tomato sauce mixture and simmer for 10 to 15 minutes or until slightly thickened.
To prepare filling:
In a large skillet, heat oil and saute onion and garlic until tender. Strain the shredded chicken from the slow cooker with a slotted spoon and stir into the skillet with onions and garlic. Add 3/4 cup of enchilada sauce and combine.
Heat oven to 375 degrees.
In a 13x9-inch baking dish, spread 1/2 cup of enchilada sauce evenly over the bottom of the dish. Fill each tortilla with 1/4 to 1/2 cup of prepared filling and roll up tortilla into a pocket. Place the tortilla in the prepared baking dish seam-side down.
Pour the remaining enchilada sauce over the filled tortillas. Cover with aluminum foil and bake for 20 minutes. Remove the foil, sprinkle with cheese and bake an additional 5 to 10 minutes, uncovered, or until cheese is melted and enchiladas are heated.
Garnish with scallions or parsley and serve with sour cream.