What is passata anyway?? I was unfamiliar with it, and really can’t name a perfect American substitution, though puree comes close.  But what I can tell you is that this stuff is USEFUL.

How useful is it? Simply reduce it by about half for an amazingly tomato-y sauce for pizza or pasta. Make a quick, real food version of America’s favorite tomato soup or use it as the base for making the best home made ketchup ever.  I haven’t tried this yet, but Pam Corbin uses it as a base for curries too.

Passata  definitely isn’t paste or cooked sauce either. Passata is simply uncooked, crushed and sieved tomatoes.  Except that Pam Corbin’s recipe (from the The River Cottage Preserves Handbook), is a little  bit more.  The first time I made the recipe I was faithful to every detail… these days I use it as a guideline. I don’t like to add much seasoning so I leave out the rosemary, thyme, basil and/or oregano. I also never have shallots; onions instead work just fine.
Once you’ve made this once, you’ll realize what a puny batch this is and you’ll want more.  Over a two or three day period, I roast as many tomatoes as I have in 2-recipe batches, saving all the roasted tomatoes in a big food-grade bucket. Then, I run them through the mill all at one time, and immediately fill and process all the jars.
For each quart (2 /500 liter jars):
2 k ripe tomatoes – a mix of several heirloom varieties is especially nice
200 g shallots or onions, peeled and thinly sliced
3-4 garlic cloves, peeled and thinly sliced
Rosemary, thyme, basil or oregano sprigs (I leave these out for a more neutral sauce)
1 tsp salt
1/2 tsp ground black pepper
1 tsp sugar
50 ml olive oil
Preheat oven to 350 F. 
Cut the tomatoes in half and place them cut side up in a single layer in a large roasting pan.  Scatter the shallots, garlic, herbs, salt, pepper, sugar and oil over top.  Roast for 1 hour, or until they are well softened.  Remove from oven and rub the tomatoes through a nylon sleeve or run through a food mill.  I use a Roma food mill, and run each batch through three times to get every drop.
Have hot, sterilized jars ready. Put the tomato puree into a saucepan and bring just to boiling point.  Pour it into jars, filling them to the brim, and seal immediately with caps, clips or screw-bands. **
Stand the jars in a large saucepan with a folded tea towel on the base or in a waterbath canner.  Cover the jars with warm water, put on high heat and bring to a rolling boil. Boil for 25 minutes, then allow to simmer for ten more minutes. 
Remove the jars and stand them on a wire rack, wooden surface or folded towel.  Tighten the screw-bands.  Leave undisturbed until cold, then check the seal. 
Use within 12 months.  Once opened, refrigerate and use within a few days. 
** Pam Corbin is from the UK and she deviates from methods endorsed by the National Center for Home Food Preservation in a few areas.  I’ve followed Pam’s recipes exactly and have had completely satisfactory outcomes, but, I’m not so reckless as to advise anyone to ignore thoroughly researched safety advice.
To preserve in shelf stable jars following the guidelines of NCHFP, you’ll need to add 2 tablespoons of commercial lemon juice to each quart. It does change the flavor a bit straight from the jar, but after you’ve made sauce or soup you’ll never know the difference.  Or, freeze instead in freezer safe containers and don’t worry about adjusting the pH at all.