Blanch 12 to 16 whole cabbage leaves in boiling salted water for 2 minutes.
Refresh leaves in iced water and drain them on kitchen paper.
Soak breadcrumbs in milk until softened in a large bowl.
Mix breadcrumbs with onion, carrot, herbs, sausage meat, salt, and pepper.
Form the mixture into 12 to 16 balls.
Wrap each meatball in a cabbage leaf and roll into a parcel.
Melt butter in a large saucepan and cook the tomato purée for 1 minute.
Stir in flour and cook for 4 minutes.
Whisk in 1 liter of stock to create a smooth sauce.
Place cabbage rolls in the sauce and simmer covered for 40 minutes.
Serve rolls hot with the sauce spooned over.