In a large saucepan, add butter, brown sugar, and onions, cook on medium high heat until onions are soft and caramelized.
Add tomatoes in a single layer, along with garlic.
Pour in enough stock to cover tomatoes 2/3 of the way up.
Stir in tomato paste until desolved.
Occasionally roll over tomatoes for even cooking, until the skins of the tomato start to fall off.
With a wooden spoon crush tomatoes, and continue to stir until tomatoes break down into bite sized piece and the stock has reduced by about half.
Now, break out the siv and put it on top of a large bowl.
Now, ladle in some of the sauce and push through the siv with a wooden spoon, occasionally scraping the bottom of the siv. You'll notice that the onion, tomato, and even the big chunks of garlic get pushed through, leaving behind the tomato skins.
Do this in batches till you've sivved all of the sauce.
If the sauce is still too thin, return to the sauce pan and reduce until desired thickness.
Turn off heat and stir in extra virgin olive oil, Italian seasoning, and salt and pepper to taste.