Preheat the oven to 200°C/fan 180°C/gas mark 6. Put the squash in a large roasting tray and drizzle with 1 tbsp oil. Roast in the oven for 1 hour 15 minutes until soft. Carefully remove the squash from the shells, leaving a 1cm border, and roughly chop. Set aside and keep the shells warm until required.
Meanwhile, put the squash seeds in a colander and rinse, removing any of the existing flesh sticking to the seeds. Place on kitchen paper and dry thoroughly. Put on a baking tray and drizzle with 1 tsp oil. Place in the oven for 30 minutes, or until they are just starting to colour. Remove from the oven and set aside.
Heat the remaining 2 tbsp oil in a large pan. Cook the onion for 4-5 minutes, or until starting to soften. Add the garlic and continue to cook for a further 1 minute or so. Add the chicken and cook for 3-4 minutes until golden. Add the smoked paprika, chipotle paste, tomatoes and chicken stock. Bring to the boil and reduce to a simmer. Partially cover and cook for 30 minutes, or until the chicken is very tender and the sauce has thickened. Using a fork, shred the chicken in the pan.
Add the black beans and reserved squash and continue to cook for 2-3 minutes until warmed through. You want the mixture to be warm but not piping hot. Spoon the chicken mixture into the squash halves. Decorate with the avocado, tomatoes, feta and fresh coriander. Sprinkle with 2 tbsp toasted squash seeds to serve.