Discard any damaged or yellowing outer leaves from the cabbage. Carefully remove 12-16 leaves (depending on size), keeping them whole. Heat a large pan of salted water until boiling and blanch the leaves for 1-2 minutes. Remove the cabbage with tongs or a slotted spoon, rinse under cold running water and then refresh in iced water. Drain on kitchen paper and set aside. This will ensure the cabbage leaves stop cooking and stay vibrant green.
In a large mixing bowl, soak the breadcrumbs in the milk and leave to soften. Once softened, add the onion, carrot, mixed herbs, sausagemeat and a generous pinch of salt and pepper. Mix well to evenly distribute all the ingredients. Form into 12-16 balls using your hands (depending on how many leaves you have).
Place the cooked cabbage leaves on a board and fill each leaf with a ball of stuffing. Fold the sides of the leaf over to enclose the filling and then roll into a small parcel. Place the rolls join-side down on a work surface while you repeat with the remaining leaves (if you have cocktail sticks to hand you can use these to ‘skewer’ the parcels which helps prevent them coming undone).
Heat a large saucepan or casserole over a medium heat. Add the butter to the pan and, once melted, add the tomato purée. Stir over the heat for one minute to cook the tomato purée and then sprinkle in the flour. Cook for 3-4 minutes. Pour in the stock, whisking to create a smooth sauce.
Place the cabbage rolls, join-side down, into the saucepan. Bring the pan to a simmer, cover and simmer for 40 minutes, or until the filling inside the cabbage is cooked through.
Serve the cabbage rolls hot or warm with the sauce spooned over.