Heat half the oil in a large casserole dish, then brown the sausages all over. Remove from the pan and set aside. Add the remaining oil, tip the veg into the dish and fry for 10 mins. Stir in the tomato purée and cook for 1 min more.
Heat oven to 200C/180C fan/gas 6. Return the sausages to the pan with the beans, thyme and some seasoning, then pour in the stock and bring to a simmer. Remove from the heat, sprinkle over the breadcrumbs, then bake in the oven for 25-30 mins until the crumbs are golden and the stew is bubbling.