Heat the oil in large sauté pan over moderate heat.
Add the chicken pieces sauté skin side for 2-3 minutes until well coloured.
Season well. Add the chorizo, onion, garlic, squash. Cook for 2-3 minutes until just coloured too.
Add the tomato passata, beans, chicken stock if using. Season with salt and pepper.
Simmer with a lid for 30 minutes until cooked through over low heat or transfer all in the slow cooker and cook over low setting for 6 hours or on high for 3-4 hours.
Check the seasoning and serve immediately.