Preheat the oven to 200˚C (390˚F). Half the tomatoes and scoop out the seeds into a bowl. You’ll be surprised how much water there actually is in them. Mix all ingredients until the tomatoes are evenly covered in oil and seasoning.
Spread the mix onto a baking tray lined with baking paper. Roast for about 20 minutes until the tomatoes have dried out slightly and the onions are cooked through.
These oven roasted heirloom tomatoes keep well for several days in the fridge when placed in an airtight container or mason jar. You can add any other seasoning you like or spice it up with thinly sliced olives or add capers.