1. Place peppers and onion into slow cooker. Cut meat into large chunks.
2. In a small bowl, combine salt, garlic powder, cumin, chili powder, and sugar. Rub spice mixture over all sides of the meat pieces, then place meat into slow cooker on the vegetables. Add any remaining spice mixture to slow cooker over meat.
3. In a small bowl, whisk together salsa, lemon juice, oil, and garlic. Pour over meat and vegetables.
4. Cook on high for four to five hours or on low for six to eight hours.
5. Serve meat with the vegetables and sauce over rice or in a taco, with avocado and fresh lime (if using).
This dish is best enjoyed fresh. If necessary, you can prepare it a day or two ahead of time, then rewarm it, covered, in the oven.