Season the chicken with salt and pepper, then add to the pan. Pour in the chicken stock and raise the heat to maintain a gentle simmer (avoid bringing to a boil). Add mashed white beans if using. Cook until the chicken is cooked through, about 15 minutes or so. If the chicken pieces are not completely submerged, then turn them over halfway. Remove the chicken and place on a plate or in a shallow bowl. Use a fork and knife to shred the chicken, then add it back along with any liquid and the green chillies. Cook until the chicken is warmed through, about 2 minutes; squeeze in the lime and adjust seasonings as needed. Ladle into bowls and top with coriander, avocado and sliced jalapeño. Serve with lime wedges.