1. Tomato Passata, known as Tomato Puree in America, is pureed strained tomatoes that is sold in tall bottles. It's 100% tomato, no additives or flavourings. It's smoother than crushed tomatoes and makes gorgeous thick tomato based sauces. In Australia, you'll find it in the pasta section in supermarkets and it costs around the same as canned tomato. Passata is not the same as tomato paste which is a concentrate.
American canned Tomato Sauce (Hunts etc) is a suitable substitute, or your favourite tomato pasta sauce (use 1 cup). Canned crushed or finely diced tomato can also be used but you'll need 1 x 400g/14oz can, add it first and cook down for 5 minutes before adding milk, cream etc.
2. The sauce thickness mostly comes from the passata so this recipe could actually be made with just milk, no cream, for a healthier alternative though you will miss the richness that cream brings to the sauce. You could also use evaporated milk, again for a healthier alternative. If you want to add richness to the sauce, add extra butter!
3. Don't try this with store bought grated parmesan as most don't melt properly in the sauce.
4. Stock powder brings more flavour to the sauce than just using salt. If you don't have it, I would add an extra 1/4 - 1/2 cup parmesan.
5. ADD-IN SUGGESTIONS: Shrimp/prawns, BACON, pancetta, chicken, turkey, diced or sliced capsicum / bell peppers, mushrooms, spinach, grated zucchini. For prawns/shrimp and whole chicken pieces (breast, thigh), cook it first then take it out, cut chicken then toss back in later with pasta. For chopped chicken, bacon, pancetta and vegetables, add them when the onion is partway cooked, then proceed with recipe. Stir through delicate vegetables like baby spinach into the sauce with the pasta.
Don't go overboard with the add-ins otherwise you might not have enough sauce!
6. Nutrition per serving, assuming 4 servings.