Add the pears and lemon juice to a preserving pan and toss together, then add the strawberries and measurement water. Cover and simmer gently for 30 minutes, stirring from time to time, until the fruit is soft and pulpy.
Purée the fruit in batches in a food processor or liquidizer with the cooking water or press through a sieve. Weigh the purée, then return to the preserving pan. For every 500 g (1 lb) of purée add 250 g (8 oz) sugar.
Heat gently, stirring from time to time, until the sugar has dissolved, then increase the heat to medium and cook for 25–30 minutes, stirring more frequently towards the end of cooking, until the fruit mixture has darkened very slightly, is thick, creamy and glossy and drops slowly from a wooden spoon. Skim with a draining spoon or stir in butter if needed.
Ladle into warm, dry jars, filling to the very top. Cover with screw-top lids, or with waxed discs and cellophane tops secured with elastic bands. Label and leave to cool.