Brush a frying pan with oil and brown sausages over medium heat for 10 minutes.
Slice celery and carrots into diagonal pieces 1.5cm thick.
Preheat the oven to 180°C.
Add onions to the pan and cook for 8 minutes until softened.
Trim fat from chicken thighs and cut them in half.
Add garlic and chicken to the pan and cook for 4 minutes until coloured.
Transfer the mixture to a large flameproof casserole dish.
Stir in gammon, vegetables, tomatoes, wine, 300ml water, sugar, spices, and herbs.
Simmer on the hob, cover, and bake in the oven for 45 minutes.
Stir in the drained beans and bake covered for another 30 minutes.
Mix chopped parsley with orange zest in a small bowl.
Serve the cassoulet topped with the parsley garnish.