PREPARE THE CHICKEN: In a medium bowl, mix the chicken with the diced tomatoes, spices, and onions together, leaving out the star anise and chicken stock. Pour all the contents into a slow cooker. Add in the chicken stock and star anise. Set the slow cooker on low for 7 to 8 hours, or on high for 4 to 5 hours.
PREPARE THE CREMA: When the chicken is about done, start making the crema. Blend all the crema ingredients in a food processor until everything is smooth. If sauce is a bit thick, add another tablespoon of yogurt.
PREPARE THE WRAPS: Rinse the collard greens with water and pat them dry with paper towels. Trim off the tough stems.
When the chicken breasts are cooked, shred them with a fork. Assemble the tacos by adding some chicken, red pepper, and red cabbage inside a collard green leaf. Top with some avocado crema.
Store any leftover avocado crema in an airtight container. It should last for another 5 days. Remember to give it a quick stir before serving it again.