Passata
Passata is just sieved tomato pulp.
It can be bought in packets or jars in supermarkets and delis; there are various brands including store own brands and prices vary. It is usually stocked near to the pasta sauces and tomato purees.
You can dilute it, add herbs, sugar, whatever to the pulp later depending on the recipe.
It is easy to make homemade passata:
Any variety of tomato will do, even green if you want tart green sauce. If you grow tomatoes, you might like to try plum tomatoes.
the amount of tomatoes you need depends on the amount of tomato pulp required: I use about 1 pound (450g) of tomatoes for a 4 person serving recipe.
if using fresh tomatoes:
pierce the skin with the point of a sharp knife, put the tomatoes into a deep casserole (pan whatever) of very hot boiled water, off the heat, on a flat surface. Cover with a lid and leave for about a minute until the skin
starts to split, then, with a large spoon to take out each tomato, peel the skin off.
You may need to change the water for fresh hot boiled water to get the remaining tomatoes to peel easily: as the water cools, the peel hardens and it becomes harder to strip off.
Put the tomatoes, a few at a time, into a sieve ( wire or plastic), placed over a bowl and use some kind of pestle ( I use the end of a rolling pin (pastry roller)); gently push the tomato flesh through the sieve by rubbing
with the pestle in a side to side motion; the pulp is caught in the bowl below leaving the seeds behind in the sieve. The pulp in the bowl is the passata.
Or you could chop the tomatoes very finely; first remove the skins and seeds.
You could sieve tinned tomatoes after removing the seeds, or tinned chopped tomatoes which have the skins and seeds removed - more expensive.
Some people suggest using tomato juice or diluted tomato paste as a substitute but I don't unless it's an absolute emergency: I think the best results are from either shop bought product or fresh tomatoes.