2 – 16oz jars milk tomato based salsa
2 tablespoons cider vinegar
5 teaspoons chili powder
1-1/2 pounds beef chuck pot roast
12 – 6 inch corn tortillas
3/4 cup sour cream
3 cups shredded lettuce
Spoon 1 cup salsa into a small bowl; reserve. In a slow cooker, combine remaining salsa with vinegar and chili powder. Add beef; cover and turn heat to low. Cook until beef shreds easily with a fork, 10 to 12 hours. Shred meat and spoon into a serving bowl. Preheat oven to 300 degrees. Stack tortillas, wrap in foil and bake until warm, about 10 minutes. Place sour cream and lettuce in bowls. Just before serving, put, peel and dice avocado and place in a bowl. Assemble your tacos.