Cut tomatoes into big pieces, bring to a boil and let simmer until the liquid has reduced quite a bit, 45 minutes - 1 hour. Pass (hence passata) the mixture through a food mill as above.

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To conserve passata: fill hot sterilized jars with hot tomato liquid and screw on sterilized lids until just closed, do not force them. Place the jars in a stock pot with plenty of water to cover completely, bring to a boil and let them simmer for 45 minutes. Let the jars cool in the water so they slowly come to room temperature. Store in a dark, cool spot. 

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