Peel the onions & garlic then slice thinly. Set the garlic to one side for now.
Next, set a large saucepan over a medium heat, add in the olive oil then leave to heat up for a minute. Add in the sliced onions & a good pinch of sea salt then cook, stirring regularly until the onions are soft and translucent. This will take 5-10 minutes & the onions shouldn't take on any colour.
Whilst the onions are cooking, pick the basil leaves from the stalks. Set the leaves & stalks to one side, keeping them separate.
Once the onions are cooked, add in the sliced garlic & cook for another minute. Then add in the tomato paste & continue cooking for another 4-5 minutes, until the tomato paste has turned a deep red colour. Make sure to stir frequently.
Next, add the tinned tomatoes, oregano, vinegar & sugar into the pan. Fill each empty tomato tin up ⅓ full with water. Use this water to rinse the tins out then stir it into the sauce.
Turn the heat down slightly to medium-low, add in the basil stalks & parmesan rind then lightly season the sauce with salt & pepper.
Bring the sauce to a gentle simmer then cook, stirring regularly until the sauce has thickened, has turned a deep red & the tomatoes no longer taste bitter. This step will take around 20-30 minutes.If you're using tinned whole tomatoes, crush them with a wooden spoon as the sauce cooks. Once cooked, take the pan off the heat then remove the basil stalks & any of the parmesan rind that's left (if there's any).
Stir in the balsamic vinegar & basil leaves then let stand for 1 minutes so they can wilt.
To finish the sauce, give it a quick blend with a stick blender or jug blender then season with salt & pepper as needed. Use straight away or transfer to a container, leave to cool down then store in the fridge.