Preheat oven grill to high, with rack directly below the grill.
Halve tomatoes and place cut side down on an oiled shallow roasting pan.
Place directly under grill on high and cook until skins start to blister and burn. Set aside to cool.
Preheat oven 230°C / 210°C fan forced.
Drizzle garlic bulb with olive oil on a sheet of aluminium foil, wrap loosely and seal. Bake for 20-30 minutes until cloves are soft. Set aside to cool.
Zest and juice orange, reserving the juice for the end of the cooking process.
Wash and dry fresh oregano and pick one cup of leaves.
Squeeze out roast garlic from cloves into food processor bowl.
Add remaining ingredients to food processor, except orange juice.
Process on high until the ingredients are fully broken down and there are no lumps of tomatoes.
Transfer the sauce to a large pan and cook on medium high heat, stirring as required.
Reduce sauce by about one-third. It should start to look glossy.
Add orange juice and cook another minute.
Remove from heat to cool and store in fridge until required.