Preheat the oven to 190C/170C fan/gas mark 5 .Place a large casserole dish on a medium heat and drizzle in a little olive oil. Fry the sausages for around five to eight minutes, until browned all over.
Meanwhile, slice the pork belly into 2-3cm chunks and slice the celery and carrot into 1cm slices, halving the carrots first if they’re thick. Cut the onions into slim wedges.
When the sausages are browned, remove them to a plate and repeat with the pork belly pieces, frying for five to 10 minutes until browned, then adding to the plate with the sausages.
Add a little more oil to the pan if needed and add all the sliced vegetables, including the garlic and the ground fennel seeds. Fry for 10 minutes, reducing the heat a little to stop it burning.
Return the sausages and pork belly to the pan and stir in the chopped tomatoes, stock and bay leaves. Season well and bring to the boil. Cover with a lid and place in the oven for 30 minutes.
After half an hour, remove the cassoulet, stir in the beans and return to the oven for a further 40 minutes, leaving the lid off. Check part way through that it’s not drying out (add a splash of boiling water if it is).
Meanwhile, prepare the topping. Mix together the breadcrumbs, thyme leaves and orange zest. Season and drizzle in a little olive oil, and toss together.
When the cassoulet is ready, remove from the oven, scatter over the breadcrumbs and return to the oven uncovered. Turn up the heat to 200C/180C fan/gas mark 6 and cook for a further 20 minutes until golden and bubbling – you could pop it under the grill for a few minutes. Serve with a big bowl of greens.