In a medium saucepan over a medium heat, add the oil, onion and garlic, and cook until they become soft.
Add the tomato passata, then reduce it by bringing it to a simmer, stirring occasionally.
Remove the sauce from the heat, pour it into a blender, add the thickplus, then blend together until the sauce becomes smooth.
Pour the sauce back into the same saucepan, add the stock and cornflour, then bring it to the boil over a medium heat until it thickens, stirring constantly.
Reduce the heat and simmer the sauce uncovered for a further 5 minutes.
Season the sauce to taste and serve it.