1. Put a small plate in the refrigerator to chill before you begin. Halve and pit the apricots. Crack a few of the pits with a hammer and take out the kernels; discard the rest. Put the kernels into a heatproof bowl and pour in enough boiling water to cover. Leave for 1 minute, then transfer to a bowl of cold water. Drain again and rub off the skins with your fingers.
2. Put the apricots, kernels, lemon juice, and 10fl oz (300ml) water into a large stainless-steel pan and bring to a boil. Lower the heat and simmer, stirring occasionally, for 25 minutes, or until the apricot skins are soft, the fruit is tender, and the mixture reduces by one-third.
3. Add the sugar to the pan and stir until it completely dissolves. Increase the heat and bring the mixture to a boil, without stirring, for 10 minutes, or until it reaches the setting point. Remove the pan from the heat and test for a set.
4. With the pan still off the heat, use a large metal spoon to skim the surface and remove the kernels. Immediately pour the jam into the prepared jars to within 1/8 in (3mm) of the tops and seal. Let the jam cool, then label and date the jars. Store in a cool, dark place until ready to use, then refrigerate after opening.