Cabbage Rolls in Tomato Sauce (Holubtsi)

Ingredients

12 cabbage leaves
1/2 cup chopped onion
2 tsp salt
1 tsp pepper
1/2 cup milk
1 lb (about 500 g) ground beef
3/4 cup cooked rice
1 egg

Sauce

1 can tomato paste
1 can chopped tomatoes
1/2 cup water
2 Tbsp vinegar
3 Tbsp sugar
2 Tbsp cornstarch mixed with 1/2 cup cold water