Season brisket on both sides with salt and pepper.
Place onion and garlic in a 3 ½-5 quart slow cooker. Place brisket on onions and garlic, fat side up. Pour Sofrito over brisket and sprinkle chipotle chili over sauce. Cover and cook on LOW 9-10 hours or until brisket is fork tender.
Carefully remove brisket from cooker with as little sauce as possible. Place on cutting board and cut into three pieces across the grain. Shred beef with two forks and return to cooker. Stir into sauce and season with salt and pepper (and a little more chipotle seasoning if you like it spicy).
Serve with tortillas and other desired toppings.
Nutrition information per serving: Calories 315; Total fat 19g (Sat. fat 4.5g; Trans fat 0g); Cholest. 90mg; Sodium 970mg; Total Carb. 4g; Fiber 1g; Total Sugars 1g; Protein 32g; Vit D (2% DV); Calcium (4% DV); Iron (15% DV); Potas. (18%DV)