Preheat the oven to 200°C, fan 180°C, gas 6. Heat the olive oil in a large ovenproof frying pan or flameproof casserole and gently fry the sausages until browned all over. Remove from the pan and set aside. Meanwhile, cook the potatoes in simmering water until just tender. Drain well and lightly crush with a fork. Set aside.
Drain off all but 1 tablespoon of the oil from the pan, add the shallots and the carrots and cook over a medium heat for 5 minutes until golden. Reduce the heat, sprinkle in the flour and cook for 2 more minutes, stirring all the time.
Pour the cider and stock into the pan and bring to the boil, stirring all the time, until the sauce has a smooth consistency. Simmer for 5 minutes until the sauce is thickened. Add the sausages and fresh herbs and season to taste.
Combine the potatoes with the apple, season and spoon over the sausage casserole. Bake in the oven for 20 minutes until golden and bubbling. To serve, simply scatter with a little fresh thyme.