Fry the onion in the oil over a medium heat until it begins to turn translucent (around 5 mins).
Add the garlic, and continue to fry for another couple of minutes, until the raw smell has gone.
Stir in the carrot, celery, rosemary, and bay leaf, reduce the heat, and gently fry for about 20 mins, until the onion becomes golden brown, and the other veggies have softened.
Make the tomato sauce
Add the passata, sugar, and salt to the soffritto, and bring to a gentle bubble. Turn the heat down very low (it helps to have a diffuser, as this will keep the sauce from catching), and leave to gently simmer for another 20 mins or so, until it has reduced and thickened.
Stir in the basil and parsley, and simmer for another five minutes.
Taste - it should be rich, thick, herby, and slightly sweet. Season with black pepper, and then use in your favourite Italian dish.
If you want a smoother sauce, you could blend it. This is especially good if using for pizza.
If cooking pasta to have with this, once the pasta is cooked, add a small ladleful of the pasta water to the sauce, and beat it in. This brings the whole thing together, gives the sauce a slight gloss, adds seasoning (assuming you've salted the cooking water), and just finishes it off nicely.