Many of our readers - especially in Britain - have taken us to task over our answer that passata is a cooked tomato concentrate. Not so, they say. We buy it all the time in our supermarkets in bottles, cans, or cartons, and it is essentially crushed tomatoes.
So we looked through more than 30 major Italian cookbooks and reference books, and guess what? No recipes whatever. Our leading food dictionaries have no entries, skipping from paskha (a Russian sweet-cheese dessert) to passion fruit. We began losing faith in the cooked tomato theory.
Finally, we came across two British books A Cook's Guide to Italian Ingredients (Canada, UK) and La Cucina Italiana (Canada, UK) that seemed to agree on a definition for passata: "sieved red tomatoes." Depending on the degree of sieving, the pulp can be perfectly smooth (polpa di pomodoro) or slightly chunky (passata rustica). So passata - at least according to some Brits - is skinned, seedless, unflavored, uncooked tomato pulp, either slightly chunky or smooth.
If you can't find it in a store on this side of the Atlantic (apparently it's just pouring off the shelves in the UK), you can drain and sieve canned tomatoes to make uncooked passata in any quantity you need. A food mill will work especially well for this.
Beyond that, dear readers, you're going to have to decide for yourselves - on a case-by-case basis - whether the recipe you're preparing calls for cooked or uncooked passata. If you recipe is from a British source, you'll probably be safe with sieved tomatoes. Obviously there is a richness and complexity of flavor in the cooked passata recipe that will be lacking if you use plain sieved tomatoes in other recipes.
In general, don't substitute tomato paste for passata unless the recipe calls for a very small amount, as tomato paste is both too concentrated and too sweet to stand in for large quantities of passata.