Line the bottom of a medium sized pot with the smaller cabbage leaves.
Sprinkle the sliced onions on top.
Place rolled cabbage on top of the onions seam side down.
Pour tomato sauce over the rolls to cover them.
Place a clean flat plate face down on top of the cabbage rolls.
Cover the pot and place over high heat.
Bring to a boil then reduce heat to medium-low to simmer 1 to 1-1/4 hours until cabbage is soft.
Check after a while, if getting dry, top with a little water.
Remove rolls from tomato sauce and place on a plate, remove toothpicks if you used them.
Using a sieve strain tomato sauce into a small pot, boil until reduced and thickened. Taste and correct seasoning.
If you want a thicker sauce, mix cornflour with 1/4 cup water and stir into the sauce a little at a time until thick enough.
Pour over cabbage rolls to serve.