To a large 5 quart pot add the olive oil and heat on medium-medium high until shimmering.
If using onions (see note below) add them here and cook on medium for 10-15 minutes until golden brown and soft.
Add the garlic, bloom for a few seconds.
Add the Italian sausage, using your spoon to break up the meat. Cook until browned. It doesn't have to be thoroughly cooked through, it will finish cooking in the sauce.Dab away or drain excess oil.
Add the 2 cans of whole tomatoes, using your spoon to mash the tomato a bit to break it up.
Add the tomato sauce, then fill the empty can 3/4 full with water, stirring to get the remaining sauce combined with the water, then add this to the pot.
Add the tomato paste and stir.
Add the sugar and oregano. Cook on medium heat until the sauce starts to bubble. Then turn it to low. If using the parmesan rind, add it now and let the sauce simmer.Stir often, scraping the bottom of the pot, then allowing it to simmer on low, ideally for 1-2 hours to develop more flavor. Can be used after 30 minutes of simmering if you're short on time.
Taste for seasoning, adding salt & pepper if needed, and removing the parmesan rind if used.