Layer half each of peppers, onions, garlic and chicken in a slow cooker; sprinkle with one package taco seasoning and 1/2 teaspoon salt. Repeat layering; drizzle with oil. Cover and cook on low setting for 4 to 6 hours, until chicken is fully cooked and juices run clear. Stir to combine. To serve, spoon chicken mixture onto tortillas, adding desired toppings.