For the sausage casserole, heat the vegetable oil in a wide, shallow ovenproof pan (or flameproof casserole) over a medium heat. Add the bacon and fry until crisp and golden-brown, then remove from the pan and set aside to drain on kitchen paper.
Add the sausages to the pan and fry until browned all over, then remove from the pan and set aside to drain with the bacon.
Add the onion, garlic, carrot and celery to the pan and fry for 5-6 minutes, stirring regularly, until softened.
Add the beans, vinegar and sugar and increase the heat to high. Bring the mixture to the boil, then reduce the heat until the mixture is simmering. Simmer for 4-5 minutes, or until the volume of liquid has reduced by half.
Pour in the wine and the stock and add the bay leaves. Return the bacon and sausages to the casserole. Slice the inner Savoy cabbage leaves (from the pesto ingredients) and stir them into the stew. Stir well and bring the mixture to the boil, then reduce the heat until the mixture is simmering, and simmer for 10-12 minutes, or until the volume of liquid has reduced by about half, and the casserole has thickened.
For the pesto, finely chop the remaining Savoy cabbage leaves and put a small handful in a food processor. Add the parsley, Parmesan, pine nuts, garlic and half of the olive oil and blend to a rough paste. Add more oil as necessary to loosen the mixture, pulsing the pesto as you add the oil.
Transfer the pesto to a jug and season, to taste, with salt and freshly ground black pepper.
Blend the bread, lemon zest and the remaining chopped cabbage leaves in the food processor.
Preheat the grill to a medium setting. When the casserole has cooked, drizzle the pesto all over the surface, then sprinkle over the breadcrumb mixture. Drizzle the breadcrumb mixture with a little olive oil. Transfer the casserole to the grill and grill until the crust is toasted and golden-brown. Serve immediately.