In a large heavy-bottomed or enamel cast iron wide skillet or saucepan, heat the olive oil over medium-high heat. Add onions and sauté for 3 minutes. Add garlic and oregano and sauté for 2 minutes.

Add tomatoes and stir to combine. Use ½ cup water to swish out the residue in the tins and/or jars of tomatoes and add it to the sauce. With a potato masher or a wooden spoon, press down and break up any tomato chunks, if necessary. Season the sauce with salt, pepper, bay leaf and basil stems (if using).

Over medium-high heat bring the sauce to a bubbling heat, this should take about 3 – 5 minutes. Reduce heat to medium, leave uncovered and let cook for 5 minutes to allow some to reduce. Watch splatters and reduce heat further, if necessary.

Reduce heat to medium-low, partially cover with a lid, and let simmer for 30 minutes, stirring occasionally. The sauce should reduce slightly and fully develop a rich tomato flavour.

Add balsamic vinegar and stir to combine. Reduce heat to low and simmer uncovered for another 10 minutes to allow the balsamic vinegar to infuse the flavour.

If the sauce thickens too much, stir in about ¼ cup of water to thin it out slightly. Remove and discard bay leaves and wilted basil stems.