Peel and very roughly chop the onions, garlic and carrots, and trim the leek and celery, then blitz in a food processor until finely chopped (if you want your sauce a little chunkier, coarsely grate them instead, using a box grater).
Heat the oil in your largest heavy-based pan over a medium heat, then add the onion mixture and fry for 10 minutes, or until softened, stirring occasionally.
Finely chop the remaining vegetables (again, coarsely grate, or roughly chop by hand for a chunkier sauce), then add to the pan and fry over a low heat for a further 20 minutes, or until softened, stirring occasionally.
Stir in the oregano, tomato purée and bay leaf, fry for 2 to 3 minutes, then pour in the chopped tomatoes or passata.
Add the sugar and vegetable stock (I try to use homemade stock, but if you’ve only got stock cubes, that’s fine too), then cover and let it bubble away over a low heat for 30 minutes to 1 hour, stirring occasionally – the longer you cook it, the more the flavour will develop as it reduces and thickens.
When it’s ready, fish out the bay leaf and season to how you like it (it’s important to season the sauce right at the end – it will thicken and intensify in flavour as it cooks, so you might find it’s salty enough already).
Divide the sauce between freezer-proof containers and leave to cool, then label and pop into the freezer for up to 3 months, to use whenever you like. This is great as a simple pasta sauce or in my lasagne, with simple spaghetti & meatballs or as a tasty base for pizza.