Put the chorizo in the base of a heavy-based saucepan or flameproof casserole dish over a medium heat. Fry, stirring frequently, until it has rendered some fat and is crisp at the edges. Drain on paper towel and set aside.
Increase the heat slightly and stir in the celery, onions, green capsicums and garlic and continue frying, stirring frequently, for 5 minutes or until soft. Remove vegetables from the pan and set aside.
Reduce the heat to low and add the oil to the sausage fat remaining in the pan. Sprinkle in the flour, stirring constantly, until well blended. Cook very gently, stirring frequently, for 5 minutes until the mixture turns a rich brown. This is the roux, which flavours and thickens the gumbo.
Add the tomatoes with their juice, the stock, okra, parsley, thyme, bay leaf and cayenne pepper. Bring to the boil, then reduce the heat, half cover the pan and simmer for 30 minutes, stirring frequently, until the okra thickens the soup.
While the soup is simmering, cut the chicken into bite-sized pieces. Increase the heat and bring the liquid to the boil. Stir in the rice, then reduce the heat to low. Add the chicken and the celery-onion-capsicum mixture and simmer for 15–20 minutes until the rice is tender and the chicken is cooked through. Pour in a little extra stock if needed. Add the prawns and reserved chorizo and simmer for 1 minute or until the prawns turn pink and the sausage is heated through.
Remove the bay leaf. Season to taste, then ladle into bowls and garnish with the reserved celery leaves. Serve with Tabasco, if you like.