Start by dicing up the onion. Or to save some time, quarter the onion and add it to a food processor along with the garlic and process until it’s small pieces like the above picture.
When the sausage is partially done, add the onion, garlic, and Italian seasoning. Continue to cook, stirring occasionally until the onion just begins to turn color, 2-3 minutes.
Add the tomatoes to the cooked onion sausage mixture along with the optional parmesan rind. Push the rind into the sauce to submerge it and bring the sauce to a simmer.
Keep the heat very low and simmer the spaghetti sauce until it’s lightly thickened and the flavors meld about 20 minutes. Stir only occasionally to make sure it’s not sticking.